If I was in his shoes, I would want to know right away how you feel. I think the best thing you can do is help him find someone to replace you as best you can.

If I was in his shoes, I would want to know right away how you feel. I think the best thing you can do is help him find someone to replace you as best you can. Obviously, not an easy task, but that's a show of friendship and mutual respect. Also, while the start-up could die without you, it could also be successful with a new person who takes your place. Even if doom is looming, you should express optimism that he can be successful and as a show of good faith, offer a signed agreement that you will not go after a cut of his future success. And most importantly, mean it.