It sounds like it's from overheating, it could also be from chemicals but overheating is much more likely. All of the information that Bogertophis mentioned is spot on. I think a vet visit is a good idea but it might be better to wait at least a couple days for an in-person visit just because he's so weak (a phone call would be a good idea). Try to avoid unnecessary handling since it will cause more stress, handling for health reasons is okay. Make sure he has a lot of cover in the tank, if he's in a clear tank covering 3 of the sides will help. Try to keep the warm side a little bit cooler than usual (maybe about 86*) so he doesn't overheat (he might not be able to thermoregulate properly).