Make sure all the skills and saves are pre-filled on his sheet
When I play, I don't always fill in what my total modifier is for each type of roll on my character sheet, and just keep track of what I'm trained in, and look up the appropriate ability modifier on the fly each time. For most people this is fine, but for someone who struggles, make sure he has this all correctly filled in ahead of time, with all appropriate training bonuses and ability modifiers added together. If something changes, like a magic item increasing an ability score/modifier, make sure it gets updated appropriately.
Help him make a chart for his attack bonus and other common d20 rolls
For something like an attack bonus, help him make up a chart showing what his total will be for each possible d20 roll. So if he has +4 Str (assuming a strength-based attack) and is a level 6 character (so half his level, rounded down, gives +3), the table would look like:
1 + 7 = 8
2 + 7 = 9
19 + 7 = 26
20 + 7 = 27
You might want to make additional tables, on different colors of paper perhaps, for other common d20 + modifiers rolls (a hide check for a rogue, will save, etc.) For someone with neat handwriting, this can fit in 2 columns on a standard 3x5 index card, and they come in multicolored packs.
Sometimes you can just tell him what the raw d20 roll needs to be
Another thing you can do once the party has deduced the AC of the monster being attacked, is just say, "ok you know he has an AC of 14, so with your +7 bonus, you need to roll 7 or more to hit," and then he doesn't even have to use the chart for that monster at all anymore. Similarly if you're asking for Wisdom vs. Will roll for a Will of 15, you can say, "you have +2 on that, so you have to roll 13 or above to succeed."
Use visual aids to help with fairly simple attack damage and other simple powers
So with the same +4 Str he will have +4 on damage. Get a poker chip or something and write on it "short sword damage" around the edge, and +4 in the middle. Then he rolls the appropriate die "next to" the appropriate poker chip, and has that as a visual reminder of what he is adding. It may be faster and easier for him to do the math if he doesn't have to hold the +4 in memory, or keep looking back to his sheet for it. Make one for each of his attacks. Some might prefer 4 dots or pips (as on a standard d6) to the number 4, to either improve visualization or to provide something to tap while "counting up". You can also do this (or custom cards) for other powers that will have a constant (but user-specific) modifier to add to the roll. The token or card could also have an indicator of what di(c)e to roll to help keep track of everything in one place.
Have a friend sitting next to him to help with some of the harder computations
If he's tossing fireballs, you'll probably want to have someone help him add that up. It could be you as the GM, but I'd probably try to arrange for it to be a friend. Ideally the friend would show him how to naturally try to group the rolled dice into 10s, and then "count up" once the shortcuts run out.
Likewise, subtracting sustained damage from hit points can slow a lot of people down. I'd have a friend help with that. (If the goal were to teach the math, I'd have him have poker chips in 10 and 1 denominations to represent his HP, and have him move aside required number of HP each time, trading in as required, but that's going to take longer and prevent the game from moving along.)
Consider a dice-rolling app
If you have the technology at the table, and he doesn't mind, using a dice-rolling app will allow all the dice to be automatically tallied for him, along with adding in any appropriate modifiers. In the case where lots of damage dice are being rolled, this may be worthwhile. Most players will find this less fun that rolling a handful of dice, so be sure to ask if having less stress to do the adding up makes it worth it for him.
Have patience and model patience for the rest of the group
Don't skip ahead. Respect his process, and make sure he gets help when he wants/needs it but is allowed to work some things out for himself as well. Wait until he's finished, with or without help as appropriate, before moving forward.
Have everyone do some of this so he doesn't feel too singled out.
So for example, have everyone make a poker chip for each of their attack types. Make sure everyone has their skills and saves pre-filled for simplicity and speed, etc.
Even at our table where we can all handle the basic math, someone sitting next to the person might chime in with the total (or difference) if they seem to be taking a while with it. It might be nice to develop a culture at your table where people chime in to help other people, though with teenagers, it might be hard for that to seem helpful and not "know-it-all" or show-offish. You can probably gauge your group best in that regard.