How big a tank is he in? Is the water temp ok [75-85]. How about water quality [how ofter is the water changed]? He may just not be hungry, how long since he ate?

Another thought is that he may need some time time to adjust to his new environment. Bettas aren't exactly the most lively tropical fish out there, not like a zebra danio.

Pretty normal for a betta to just kinda hang. 'Not lively' is a charitable description - they're downright motionless a lot of the time.

It's hard for us to visualize how little food a fish needs. They're cold blooded so need only a small fraction of the amount we would need at the same body size. A rule of thumb is no more than the size of their eyeball per day, but mine thrives on way less than that. I have betta pellets for him, little tiny grains really, and he gets about two a day and nothing on Sundays. They don't need to be fed every day.

He's also in a two gallon bowl, the smallest thing I'd keep one in.

He's thriving, and I've had him over a year, and he spends most of his time just hangin'. That's normal.