I assume he won't offer you real money. ("Royalties after it sells" is nothing -- you'll never see a penny.)
Decide beforehand if you want to do it for no money or not. Know why you're willing to do it, if you're willing to do it.
If he doesn't have a collaboration agreement that clarifies ownership of the game and its assets, don't work with him. http://www.sloperama.com/advice/article58.htm