Responses above are all pretty good. Two things that weren't mentioned much: 1) How much money have you spent to date. 2) How many hours have you spent to date. Consider # 2 in the context of a four year vesting schedule... after 4 years, you may have spent 1.5x as long as him, meaning you would deserve a share 1.5 times as high as his based on this alone (valuing your time equally). You would obviously start somewhat more vested than him. Now consider how much cash you've invested personally, and how much time that represents. Frankly, I think cold hard cash investment should be the biggest reason for a difference. If you've invested say, 80K, you might offset that against at least 2000 of his hours, for example. And it also depends whether he WILL be paid a salary in the future. You probably want to play around in Excel to test some numbers, that's the fairest way, but takes some financial skills.
Full-time vs. part time is also important. If he's pledging to work full time and foregoing a real salary, he deserves more risk compensation than someone who is working part time (not just based on the rate at which they contribute hours, but from the risk perspective). One alternative that you might consider is equity plus deferred debt. Like, 10% + $X per hour accrued in debt that is payable upon revenue or financing in excess of $Y dollars. That would allow him to monetize a bit of his stake before selling the company (or going public, or getting paid dividends).