With the release of Volo's Guide to Monsters, you can now play as a Hobgoblin at the discretion of your DM. Hobgoblin traits can be found in page 119.
I'm going to teach you how to fish, here.
Firstly, you should look this reddit post up and the google doc it links to. It shows the work done by /u/JamesMusicus and /u/Aranim, and I have been using it for homebrewing balanced races myself. Even if you don't find a particular ability (like a Hobgoblin's Martial Advantage ability) there, you will likely get a feel of how much an ability should be valued as.
An example of the Abilities that are valued are:
- Ability Score increases are valued as 1 per score increased.
- Fey Ancestry (from the elves and Half elves) are given a value of 0.5.
The googledoc also goes on to say that most official races are valued at 5.5 to 6, with Mountain Dwarves at the highest (8) and Dragonborn at the lowest (4.5). (And here I am playing a Dragonborn Bard)
There are many more abilities that are valued there. So take a look at that as well as the DMG for creating a race.
A little warning
Hobgoblins are boring as a race, they only get Martial Advantage as a racial feature on top of its ability score increases and even if you throw in the Leadership Action from the Captain's statblock, it still won't be as good as, say, a Variant Human or a Tiefling.
There is also no correlation to a monster's CR and PC level so a CR 3 creature is not a 3rd-level PC as the CR 3 creature is a Medium difficulty encounter for 5 3rd-level PCs while if you tried to pit a 3rd-level PC as an encounter, it'd end up short.
In addition, if you use the tool linked above, and the NPC Features table found in the DMG page 282, a Hobgoblin race would have a value of (count it) around 3.5:
- No Ability Score Increase (page 282 of the DMG shows hobgoblins have none)
- 0.5 for Darkvision
- 2 for Leadership (and I'm being generous)
- 1 for Martial Advantage (again, being generous)
Enabling your player to play the hobgoblin captain makes sense story-wise but the race inherently doesn't give him the value a normal PC race should have. I suggest letting him play it for the mean time and have him roll up a new character as soon as an opportunity to introduce one arises.
Otherwise, if you really plan on latting the Hobgoblin stay for good, you could add a total of 3 Ability Score Increases (it's up to you which scores to increase but if it were me, those would be 2 Strength and 1 Charisma. There is no correct answer here, it's just a suggestion) and do away with the Leadership trait (Optional but if you decide to keep it, the score will go up to 6.5). You also want the Martial Advantage trait to scale to level (the Rogue's Sneak Attack feature would be a nice basis but then there's the question of "What if this player or another player who wants to play a Hobgoblin grabs a rogue level?") That will bump the score up to 4.5 (Hey, it's as valuable as a Dragonborn now).
Then you have to think of the Hobgoblin's role in society: Will common folk welcome a notoriously evil race to walk among them (Drizzt comes into mind)? Will nobility or people of power get over the fact? How does this character handle potential racism directed towards it?