If you draft a contract, I have some suggestions. First, try and keep it in English. Nothing looks dumber than a lay person trying to spiff up a document by using legalese. Second, it is absolutely essential that you write down the procedure for buying the other guy out. If you can't afford to buy him out immediately, then specify that payments will be made over the course of two years or whatever. That leaves you with only the problem of how to value the business -- and that can be a big subject. Finally, you might consider requiring joint signatures on checks over X dollars.
I presume you know that you'll be this guy's partner and partner's have a substantial liability for partnership debts. And "partnership debts" means that new Porsche that the guy bought in the partnership's name so he could make faster deliveries.
Best wishes. And remember, an oral contract isn't worth the paper it's written on.