I don't know a ton about bettas, but I do think if you were able to get him a bigger tank, then he'd be happier. I think I read somewhere that ideally they'd be in 3-5+ gallons. I also think they're supposed to be 76-80 degrees water, but I imagine room temperature is near that so I don't think it's that.

Could it be velvet? I'm definitely just googling things, sounds like velvet can cause loss of appetite.

How long have you had him? Sometimes animals won't eat when you first get them, and I think that's universal among most pets. Raichu didn't eat for two weeks when I got her! I was so nervous but it was normal.

tbh i used to have a bigger tank but my old betta had something wrong with her fins and had a hard time swimming to the top so i got a smaller one so she could eat and stuff without like. really straining herself. we literally never found out what was wrong with her either??
i'll have to see if i can find some money to get a bigger one for this guy eventually. i think he might just be having a hard time getting used to things. i haven't even had him a week yet. and i don't think he's sick or anything either bc the whole reason i picked him was bc he was the healthiest looking fish there.

tbh I don't know anything about betas either dude but like i totally agree with dean

it might just be that he's nervous bc of a change??
when i first got zayn he didn't eat for like a week and a half :0

yeah i'm gonna give it some time or hopefully i'll get some good tips next week and i'll grab him a bigger tank after work ;u;