The question you are trying to answer isn't 'what should I make him pay', it is 'what is my business worth?' vs 'what would good business be worth without mine?' in terms of present value.
Once you have that present value, the other question answers itself and there would be plenty of formulas online to help you decide on a specific number.
So how much will he make, extra, because he can use your model, brand + customer list? That's the value you are selling him.
Is he completely dependent on you? Can he copy your path on his own and get to the same place soon? How much does he need you? Is he very entrepreneurial or does he need you to do the big picture strategic thinking and he just wants to execute?
Fyi if you were talking about selling for 7000 as too much... Your business would be forecasted to generate almost 300,000 in profit over five years. $7000 would be very, very low bordering on charity.
Given the model I think a small royalty is entirely reasonable and imposes no immediate financial burden on your guy. Record keeping and trust becomes a big issue though.
But you are giving this guy a proven model and a long list of clients. Hes just doing grunt work, it's a good deal for him either way.