He is very thin... I hope you don't lose this guy.
I have had lots of experience with trouble feeders at the Pet Store I work parttime at. This is my advice to you..
Go to Walmart and get a 6 quart tub All you want in there is a tight plastic hide on the warm end and a water bowl on the cool end. Paper towel as substrate. Either buy a new UTH or put the tub in his current enclosure right on the glass over the UTH. (Making sure half of the tub [the side with the water] isn't over the UTH) Fiddle with your thermostat so inside the tub he's at 90-93 warm spot. Leave him be with NO contact for 2 days then defrost a pinky and assist feed him. If he refuses the assist feed, force feed. I'd do this every 3 days for about 5 feedings. Then I would get a hopper mouse (LIVE) and leave it in his tub overnight. Hopefully he takes it. If not, continue assist feeding and alternate from the live hopper to assist pinky.
I wouldn't go to the vet just yet...