If the poop is stuck because there's something attached to it, like a strand of hair or string, then try snipping as much of it off as possible, but don't pull on it.
If the poop is stuck because he's dehydrated and it's probably the urate that's stuck inside of him, unfortunately you'll have to let him work it out by himself. Try the warm bath soak, but if that doesn't work, try giving him 0.25 ml of olive oil (as a lubricant).
If all else fails and it'd been a few days, a vet visit is your only option.
Any updates for us?
It also could be a sperm plug or a seminal plug possibly. As suggested, try the soaking in warm water just do not pull anything that is firmly attached or you could hurt him.