20 gallons for a baby is often too big, I would downsize him to a 6 quarts tub with aspen as substrate, a 6 inches plastic flower pot saucer for hide, 78-80 on the cool side and 88-90 on the warm side.
I would let him acclimate for a week and then offer a LIVE hopper mice.
Your BP not eating IS likely husbandry related and a few changes will help solve this issue.
Also NO handling until your BP eats with consistency.
Make sure to measure your temps with a DIGITAL thermometer and make sure the temps do no fluctuate too much.