I've played D&D for years in a group containing several players who identify as having ADHD.
I've noted some challenges below that you, as the DM, might be facing, and some things that the player might consider, and some things you the DM might consider.
Given all the comments requesting more information, it may be that your question lacks information for us to be truly helpful; nevertheless, I've given it a shot. It also may be that asking a separate question may be more helpful for you. For instance, if you think managing new players is a challenge, then maybe that is a separate question.
I am making the assumption that the player is managing their attention issues as best they can outside of the game: consulting professionals as appropriate, taking or not taking medication as appropriate, and is using whatever life skills they've developed, and is still struggling specifically with playing D&D, so I've focused on game-specific topics in this answer.
Challenges
You're facing some challenges that might affect any D&D group, regardless of attention challenges, which might be making your situation harder to manager:
Large group
Six players is a lot. It is entirely doable, but it requires extra attention on the part of you, the DM.
You need to be particularly attentive to making sure that things like combat management go smoothly. You should examine each individual area of the game that you manage, and see what you can do to improve that area.
If there are specific areas that you think you need to improve, you can focus on those areas, perhaps including asking separate questions here.
In my group, our current DM says five players is ideal. I don't know that I particularly agree, but for sure all else equal five or four players is easier to manage than six. I doubt booting a player is the answer, but should one of your players leave, you might consider holding off on looking for a replacement.
Short sessions
Sessions of two hours are challenging. In our current game, our sessions are typically 3 1/2 hours; over the life of my current group, we've often had times when five hours was more typical. (Back in the day, we sometimes played 'til dawn, but that's another story entirely . . .)
You need to be particularly attentive to time management. For instance, be sure you're getting started on time. In my experience, in a group of six, often at least one individual is late or needs to leave early. Be prepared to deal with this. Or you may be struggling with side conversations, or other similar issues that are sucking the life out of your two hours.
New players
You have new players. That is fantastic! It is also an added burden. You need to be particularly attentive to new player management. You might wish to address new-player concerns as much as possible out of session. I can't emphasize this enough. Anything you can address with characters ahead of time can really help. It's possible something like an electronically available one-pager of hot topics with pointers to the rules would be useful.
Suggestions for the player
Some suggestions for the player working to stay focused:
Take notes
Maybe the player can take notes.
In our current game, I (a player) take notes and publish them each week. I take very basic bullet-based notes, review them afterward, and post in our Discord server. I record each time the session number, the session date, the dramatis personae (the person's name, their Discord name, their character name, if they were absent I still record them), and a bullet list of things that happened. I focus more on getting them published before the next session than perfection. This could be a whole question and answer by itself.
I find that this is tremendously helpful for me personally, and is also helpful for the other players. I find that it helps keep my own attention focused on the game.
In the end, everyone finds their own solutions, but this has been incredibly useful to me. I've never been one of those natural note takers, and it has taken a long time to figure out something that worked for me, but it is amazingly helpful for me personally.
Review ahead of time
Maybe the player can spend an hour some time before session reviewing character sheet, notes, references, anything.
I find this is also tremendously helpful for me. No matter how busy I am, I try to spend at least 30-60 minutes reviewing something during the week. Worst case it's just posting the notes, but I usually have at least a few minutes to review notes, references, or my character. Just reviewing the character is helpful. At low levels the character is new, and maybe needs more detail; at high levels characters can be wonderfully complex. Even the simplest character has some complex parts. I find it helps my own focus tremendously to review those things ahead of time.
A word from a popular Youtuber
Shortly after I wrote this answer, popular D&D Youtuber Ginny Di came out with a video entitled "The struggle of playing D&D with ADHD". I don't believe I can summarize in a way that does her credit, so I'll just say it's worth taking a look. Also, there are some links that might be useful below the video.
A personal note
I'm not sure if this is helpful or not, but I offer the personal observation that "ADHD" is a label; it is not you. Consider that label as a tool. When it is useful to you to apply that label to yourself, feel empowered to do so; when it is not useful to you to apply that label to yourself, throw it over your shoulder.
Suggestions for you, the DM
Some suggestions for the DM to help keep players from losing focus, particularly in remote games:
Do a recap
Try doing a group recap at the beginning of sessions.
At the beginning of each session, our DM asks a player to volunteer to recap the events of the last session. If they're even reasonably correct, the DM rewards them with inspiration.
Ninety-nine percent of the time I've posted the notes from the last session. It helps even the most attention-challenged to do the recap or to read along. Since I wrote the notes, I only do the recap if no one else volunteers. I also try to make any minor corrections to the notes.
Provide names
Provide your players with names in print.
It is amazing how easy it is to mangle fantasy names. Our group has a standing joke about it. Our DM uses a channel on our Discord server to copy/paste various character names, monster names, place names, and the like. It really cuts down on the distraction for players not to be saying, what was that, how's it spelled, did they say mindflair?
Provide visual aids
Provide your players with pictures.
Our DM occasionally posts pictures of monsters or places in a channel on our Discord server. It can be really helpful to have a visual of something in the story.
Use a VTT
Use a virtual tabletop.
Even if it is just for characters. You probably(?) already do. Maybe D&D Beyond, or Roll20, or Foundry. It's worth it. If your players are keeping track of characters on paper, that is enough to distract anyone.
Use maps
Show maps to your players.
We currently use the map feature in D&D Beyond. We've previously used Roll-20, cut-and-paste, and just sharing a screen. We've used pre-made maps, made our own, or just used diagrams. Having maps helps everyone to focus.
Record audio
I do not have personal experience of this, but some DMs record the audio of a session, and send it out (credit to CTWind). For some that's a huge lift, and the DM's already doing a lot, but for some DMs, it works.
