Try adding some indian almond leaf into the tank - it works wonders. Also try using a mirror to play with him a couple minutes a day. Don't worry about the mirror stressing him as long as you only keep it to a couple minutes a day. If your filter creates a lot of current, slow it down enough so that he can build a bubble nest. Ideally, this would all stimulate some territorial/breeding behavior in the male. Hopefully, he'll build a bubble nest and patrol his territory - giving him some purpose and something to do.