PLEASE HELP! Catfish belly side up but NOT dead!?

I found him a few minutes ago. He's a red-tail catfish about 15lbs. He's been stressed lately from a recent move and new tank. But that was months ago. He used to eat a few weeks ago but only if I hand fed him. I put in some feeder fish and he doesn't even acknowledge them. That was 2 days ago. Now before Bed I checked up on him (he has his own room and giant tank) and he's completely flipped over, belly side up! I thought he was dead but I can still see his gills moving and I rubbed his belly lightly and he began to move his tail side to side. He's still alive but whats wrong with him?! He's never done this before! I tried to help him flip over right side up but he doesn't seem to want to. Please help me, I am so worried!