I'll jump on the bandwagon. Forget him.
Here's my thinking:
1. If you don't know enough to run this business without the guy, you don't know enough to open it in the first place.
2. Not questioning anyone's honesty, but 10% isn't much incentive, and IF the place is successful, and IF, like so many small businesses, it's mostly cash, you're just building a scenario for disaster.
3. Valuing small businesses is extremely difficult. Unless there's real estate involved, or your lease becomes immensely valuable (you have a ten-year lease with a ten-year option at a very low rate, and suddenly find out that something - road improvement, new building, etc - is happening across the street and your traffic flow will increase 5000%), the major value is usually in cash flow.
4. If your partner-to-be becomes the main appeal of the business - like a fabulous maitre d' - he's got you over a barrel.
5. My recommendation (I'm assuming, perhaps wrongly, that you're going to be at least a partially absentee owner): spend a lot of time studying the kind of business you want to open. Hire a manager, after vetting him extremely carefully. Learn, from your accountant, what kind of daily, weekly, and monthly reports you should get to be able to keep a handle on business. Drop in frequently, and unexpectedly. Make sure the inventory actually exists. Make sure the restrooms are squeaky clean. Ditto the staff. Do some comparative pricing on purchases.
6. If you still need help, look for a consultant. I'm not fond of them, but at least you know your liability.