Given the lack of other data (what would be his role in your startup, how good is he at multitaskig, what are you building, how much time will it take, can you augment his absence with another person, is this Forever or For a While), it's really hard to make any good recommendations.

Some people will tell you they did this and it worked out fine, others will tell you it was the cause of their demise.

I don't mean to attack your question, it just seems that you're asking for an answer to a multi-variable problem, and you've only given 1 constant.

Also, what would happen to that $2500? Is it to pay his bills, or fund your venture?