What are his symptoms? How long has this gone on? If you've just noticed trouble for a day or so you could try upping the basking temp, but if he's been off for longer than that I would have a vet check him over and give you antibiotics. If he's not spending more time under his basking light now (often a sign of respiratory problems), he may not respond to increased temps and could get burned, so be careful.

As we don't have all the details of your husbandry and his past history it will be pretty hard to rule out other problems that might be present.