I don't see a problem with the percentage, HOWEVER, are you going to own it at the end of the 5yr period? If so, it's a great way to buy a business with nothing down, in effect using sweat-equity. If this is the case, before you agree to the deal get an attorney to generate a buy/sell agreement that spells out the terms in detail. The last thing you want to have happen is to work your butt off for 5yrs only to find he still owns the business and you're still the employee.

Don't go any further without an attorney...if he balks at this, you should have doubts about how good the deal really is.
Good luck
Denny