All sorts of problems can crop up if you don't resolve the equity situation up front. Its good you're trying to resolve it as soon as possible.

Depending on how much thought leadership he's provided and how much direction he's given you, it should range from 50/50 in the very best case for you to 80/20 in the worst case (assuming you if you're the #2). That split also depends on whether he is paying you or not to do the work, and whether you really want to loop in with him long-term on the startup.

Are you guys incorporated? Regardless, make sure that the terms are in paper.

When making equity decisions with my co-founder, I found these posts pretty valuable: http://startuplawyer.com/incorporation/how-to-split-the-star... http://startuplawyer.com/startup-issues/keep-your-startup-co...

Hope that helps.