What is the best way to stake someone?

I have a friend of mine who wants me to stake him to play no limit holdem at the casino. He is a big nit and he says if I give him $1500 to start he will give me 25% of all the profits he makes for the next 4 months and my initial investment back. How would I go about calculating his wins to losses and how much he takes.

Obviously if he takes 75% every time he wins and doesn't factor it against his losses I am losing huge on this deal.