Get him out of the pebble-filled tank. Rocks are a breeding ground for bacteria and are freezing cold if exposed to a cold room.

I would go get a 15qt tub, get one hide, put some paper towels in the bottom and put the UTH on half of the underside. Get a DIMMER to regulate the heat and measure it with a digital thermometer on the bottom of the tub, underneath the paper towels.

You want the temperatures to be between 92-95 on the warm side and 82-84 on the cool side. If you can't get those temperatures with your current set up, change it. Tubs are easy because the plastic holds heat like nobody's business.

Get a small hobby soldering iron to melt a few holes along the sides(not in the lid). I generally do 5-7 holes on the long sides and 4-5 on the short sides.

Once you get the temperatures and everything set, stick the snake in, make sure it has fresh water in a small crock bowl, close the lid, secure the lid, cover the whole tub with a dark pillowcase in a quiet part of the house and LEAVE IT ALONE. You need to leave the snake alone for at least a week, just check on it every two days or so and see if the water needs changing.

Then let it alone for another week.

After two weeks is up with NO handling WHATSOEVER, go get a LIVE small adult mouse. Put the mouse in a secure critter keeper or cage with food/water for 30 minutes to an hour. Make sure the room is quiet, dark and with NO traffic.

When this hour is up of pre-scenting, get the mouse out, drop it on the opposite end of the tub that the snake is on and CLOSE THE LID. Cover the tub with the dark pillowcase and LEAVE. Wait 30 minutes to an hour and see how the snake is doing. Well-fed mice will not do anything to the snake.

If the snake doesn't eat, remove the mouse and try again next week with the same procedure. Do not handle the snake unless absolutely necessary and keep it covered.

I would also have a vet check for internal parasites by getting a fecal done. Shouldn't cost more than $20 and deworming meds are very inexpensive.

But in all honesty, the snake needs to be out of that 20-gallon freezing cold tank. The cold pebbles aren't doing a whole lot of good either. Get him in something small such as a 15qt tub(I have pictures if you need them). It will help him feel alot more secure and not out in the open. The tubs are more like burrows to them and will help him get happy about wanting to eat.