Vet visit.

Babies can go downhill quickly because reptiles tend to not show anything until it's severe. You'd need precise temps especially on the hot side, and a lamp tends to overheat things easily.

If he'd never fed(was sold freshly hatched) he probably wasn't started correctly which makes it harder to get them to eat. It doesn't sound like IBD to me, but you need a vet visit ASAP.