My HM betta has always been like that. I think his fins are just too heavy. He mostly just stays in one place, loafing on a plant. My CT bettas on the other hand are VERY active. All snails must be flared at and intimidated until they fall off the glass in submission.
Yeah, I think he was just more active when he had something to chase. He will perk right up when I sit and watch him and is active when he wants to, this is the first time he has been by himself.