Yeah, he could just be taking a couple days. Look into baffling the filter, and if he's still tilting, it could be SBD so just fast him for a day or two and see how that works. My CT is pretty droopy when he's not flared, so I know exactly what you mean... their fins don't really move up or anything when they swim sometimes, they just kinda sit there eh? haha. But as he gets used to the new home, he'll start to flare more. Do frequent water changes too, to keep him happy, and keep him company if you can. As he gets used to you as well, he'll perk up :)
If you really want him to flare though, you can take a mirror and put it in front of his tank for a minute or so... if he approaches it and sees himself, he should flare. If he doesn't flare, he may be sick. Do not allow him to attack the glass though! He could hurt himself if he repeatedly does that, so be there the whole time ok? If he rams it once, or looks like he's going to, take the mirror away right away! He probably can't hurt himself unless he rams it like 20 times... but just to be safe haha.