Investing in someone

Hi, I've never dealt with this situation in anyway so I have no idea how it works. Basically there's this guy trying to get a start up business. He needs 5k in investing and I'm interested. I'll get 40% of what he makes every month if I invest. He'll be starting out at AT LEAST 2500 a month which starts me out at 1k a month. Now if he gets really good at his job, and he is a go getter type of person, he can make 100k a year or more. Our deal would be I get 40% of him for life. So if he makes 100k a year I'm getting 40k a year which is amazing. The problem is its toooo good. It will definately be written down in paper, but I'm not sure how I can protect myself. When he becomes a big shot 100k a year he sure as hell isn't going to want to give me 40k a year every year. Whats stopping him from closing this business and opening a new one doing the same thing. How well can I protect myself?