edit: wrote below before seeing Greg's response... the care sheet is a great place to start!

Get a thermometer and put it in the tank. Any small thermometer will work. They sell some small digital thermometers at walmart for around $10. Then you can see if you can leave the light on longer for him. You want a temperature gradient with a basking spot not warmer than 90 degrees.

His constant fish diet is probably to blame, but it's hard to say for sure. Hopefully Rhea can point you towards a good herp vet in the area. Here's some info on Thiaminase (B1 deficiency). It's an old article and the links are dead, but still good info: The long story on Thiaminase and vitamin B1

In the meantime try using feeding guppies and no goldfish. I would try to get him to take a pinky mouse, too. If he's used to fishing for his dinner try dropping one in the dish of water with a couple of fish and he'll likely take the pinky and the fish. If he spits it out the first time, don't worry. Try the same process a few days to a week later. Sometimes it just takes them a few tries to get used to the taste & texture.

Wishing you the best with the little guy.