From you submission, I am unsure whether you are joining as a member, whether he is hiring you as an employee or whether the business structure you both have set up makes you as a co-owner.
I am unclear if you are assisting him in building his business. Are you a contractor, a consultant?
However, having said the above, you know your worth and the individual hiring you should also know your value as well as what the current salary is of a “fitness coach” in your area. If the business is owned by the individual presenting the offer, he/she should have included the “salary budget” in his business plan.