Betta Lethargic and Not Eating, other causes?

Everything I have read so far makes me think the symptoms as bloat, but he doesn't look bloated to me.

We have had Grisito for nearly a year and he had always been very active and at the top of the tank hoping for food when we are around. We took a vacation and returned last week to a lethargic Betta with no interest in food. He stays mostly at the bottom in his castle or resting on a leaf. Even when we get him to the top he's had no interest in food.

Water seems good, we did a 25% change before we left and as soon as we returned, 0 ammonia and Nitrites, 7.2 ph, 75 harness. Water temp is a little lower than normal at 76. He is the only fish in a 3.5 gallon tank.

I tried an Epsom bath 2 days ago (1tbs/gallon and a acclamation tank after) but it hasn't made a difference. I've read recommendations for high fiber food but he won't eat a thing.

Usually he eats 2-3 pellets every other day but I'm not sure if that was followed by our pet sitter.

Photos are not the best because he just doesn't want to come out. Not noticing lifted scales, arched back or laying on side. He doesn't seem distressed or even very bloated. What is going on!?

I can't tell anything form those pictures.

Did you get someone to feed him while you were away?

I would do a 75% water change and gravel clean the substrate every day for a week.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Increase the temperature a few degrees.

Offer him some live food (brineshrimp, mozzie larvae, daphnia) and see if it gets him eating.

One thing I thought of is the water temp has dropped a bit, 76 when it used to be 78. Replaced the heater, fingers crossed. I did immediately notice him swim up to be closer to the new heater

It would seem that Grisito is a finicky little Betta and that 2 degree drop in temp set him off. The tank is now up to 79 and he is swimming around, coming to the top. BUT STILL NOT EATING! He comes up to his food but instead of nibbling he blows a bubble. Any other ideas what to do?

Try feeding him something else like live food. Also, I find 3.5G tanks lead to lethargic bettas because they get bored quickly. You could look into something a little bigger. It's also harder to stay on top of water quality in such a small tank.