yep.. take the heater out.. the heat may be a large part of your problem.. Just unplug it and let it sit in the tank 24 hours then remove it completely. This allows the water to slowly drop in temp and the heater.. so it doesn't explode when you remove it from the tank ( yes I've seen heaters explode). Bullheads ime also like dim lighting and aquatic plants/caves to hide in and among. an untreated flowerpot would do well to offer him a place to relax. Stress may also be a culprite. If he was wild caught you may also be dealing with internal parasites. depending how long you've had him, they can take awhile to manifest themselves inot a problem.
1) unplug heater
2) get a water test done and list perameters
3) get him something to hide in if he already doesn't have something/dim lighting
4) what are you feeding him?
and it is possible if he's wildcaught and has run a few seasons wild you will see decreased appetite, lethagic actions in prep for season changes. If he seems ot be openeing his mouth alot and "gasping" it may simply be the heat is finally getting to him. or Your water quality.