1st Pay him what you owe him...
If ya'll haven't already had a talk, now's the time.
I use a couple of Subs regularly. We have a good understanding about this kind of thing. If someone approaches him about a fence while he is working on one of my jobs, he hands them my business card.
If they pressure him to go around me, he politely tells them that he's not willing jeopardize the relationship he has with me, for one job.
If he refers them to me and they buy, I make sure he gets to install the job, and also give him a little extra $$.
He knows that I take care of him, so he takes care of me.
If he's not will to take care of you, send him down the road.