There is a test I do that usually tells me if my fish have parasites or not... It's not guaranteed to work but it can help.
When you next do your waterchange add 1 tsp of aq. salt per gallon. If he's twitching, zooming around the tank like he's agitated, trying to cough something up, or rapidly shaking his head (or body, sometimes), those are good signs he has parasites.
Update us if he does these things, please! The treatment's pretty easy.