If you have other fish in your tank he may just be a little bit shy or just wanting to check out his new hangout, but I would still watch him carefully. Also check to see if his breathing is labored and gills are flapping rapidly or if his eyes are normal and not protruding from his head. Test your Ammonia, nitrites, and nitrates and post them here as well and that will help the more experienced aquarists.
I would also suggest maybe getting some more platys if you don't have any right now (after you are sure he's ok health wise), and that may make him feel more secure.
I wouldn't take it lightly though. Not to worry you because it's unlikely, but my new swordtail I've had for three days was a little bit of a hider, and just a few hours ago his eyes swelled up, he bobbed along the tank, swam around spastically and then dramatically fell to the bottom and died. Slightly traumatizing.
2g eclipse hood, 1g hex betta, 10g planted (soon: dwarf puffers), 20g empty (soon: guppy mania tank), 29g eclipse hood empty, 55g community tank