First thing I'd do would be walk up to him and politely introduce myself. I'd explain the problem and request that you, him, and his boss have a little conference imediately. (Pre-plan this so that his boss is at hand and their is no excuse.) Stay cool. Be rational and reasonable. Make up fron demands for an apology to you and assurance that it will never happen again. You may want to talk to a lawyer before hand - it dosen't really matter what he says. In the meeting with the guy and his boss you can casualy mention discussing the matter with your lawyer. (Even if you haven't, and as long as you little gym meeting is going your way, which it will, you can still mention having talked to your lawyer just to make them shit.) By the end of the meeting the guy should be sucking your dick.