You said you only had him 4 days, yes?
I'm thinking maybe he is still adjusting to his new home. Whenever I get fish from petco, their ammonia levels are always through the roof and it takes them a few days to recover, especially if they had ammonia poisining.
he seems unable to keep himself up
like unable to swim to the surface? Or having bouyancy issues?
whenever I had a betta that was lethargic with clamped fins, something in the water chemisty was off. Ammonia was too high, or something. I use stress coat as a water conditioner and that seems to work well.