He sounds as though he may either be impacted with stool or its an abscess brewing. I would try soaking him in warm water for an hour a couple times a day ...depending on his size, big enough that half his body width fits and 2/3 of the length....making sure he's in a warm area with a heat lamp afterward and see if that helps...if he doesn't pass stool in a couple days a vet visit is needed but soaking usually will do the trick...
Let me know if you have any other questions
Good Luck &
Best Fishes