Thanks,
The best you can do is take a look at how much is him helping you with the startup and you can somehow guess if he's going to leave the project anytime soon. (obvious)
Let's suppose you are giving X% equity for him and in your eye and your startup is worth Y atm(you should have that in mind?). If X*Y(the amount of "cash" you are giving him) is BIGGER than the value that he will bring for your company(be sure of it!), give him that equity!
Also, don't forget that most of people take a lot of influence from others, so... if he's THAT kind of developer(or anything else) that could make your business fly, be a leader(in the good way) and motivate him to devote to your cause.
I'm planning to write an article soon about my experiences doing that. Your question just reminded me of that.