That's good you've got an adjustable heater. That will help keep the temp stable.
Are you absolutely totally positive that he's getting skinnier? :( Fish can go for up to two weeks without food. If it's true that he's getting skinnier in just the short time you've had him, then there may be more going on than just adjusting to a new home. For now, keep up with the water changes. If we can eliminate dirty water as a cause for anything, it will make it easier to diagnose if something is
Yeah, the whole salts thing really can be confusing. When I first got into fish, all I knew were aquarium salts so when someone said epsom salts my mind flipped. I believe the rule of thumb is aquarium salts for bacterial/fungal problems like fin rot and ich, epsom salts for constipation.
Has his breathing just now started getting more rapid or has it been like that for a few days? His gills could be a little inflamed still from whatever water he was in while at the pet store or wherever you bought him from. Keep an eye on it and if it seems like he is really struggling to breathe and is rising to the surface a lot to grab air, then do a water change.
Yup, my girl still clamps up when she gets spooked by something, sometimes pretty tightly. I've had her about two weeks now and she's almost totally adjusted but evey now and then big things scare her, like my cat landing on the desk next to her tank or me trying to take a picture with the camera. The moment I point a camera at her she gets stress stripes. Talk about camera shy. So it's possible your guy is still very stressed from his move and it may take him a while to unclamp.
The problem with fish is one symptom could mean any number of things, some harmless and some not. It's very hard to tell. Like in humans, a cough could mean someone is just clearing their throat or they could have tuberculosis. In a fish, clamped fins could mean just scared or really sick. It's actually maddeningly hard to tell. Hang in there.