You can place a small mirror in front of him, sometimes they will respond to their reflection aggressively.
I find the best way though is let them see another male (in another tank obviously!), then you really see some effort in the flare!! lol Just place two male tanks side by side that aren't used to each other's presence and as soon as one spots the other, the competition begins!! I do this with my 2 boys every so often, it seems good for them, as after 5-10 mins they go back to their usual tank positions and they both think "I WON!!" and no harm came to them, plus they fully exercise the muscles controlling the fins.