A couple of options come to mind.
First, you could tender the $15,000 along with a written offer to purchase his portion of the partnership for that amount. If he refuses, you may be able to sue for breach of contract. Be prepare to prove the value of the business, which is most likely the value of the assets less the liabilities - not the annual revenue. One of the interesting things about your partner's argument is that he thinks he causes the business to have value, but wants to be paid for that value, even though he won't be there after the sale. If he's really the most valuable thing about the business, he can't sell his interest in the business for what it's worth with him in it, because he won't be there anymore.
The other option is to let him buy you out for $30K.