I'm sorry Jacki. I would try an immediate, emergency treatment with antibiotic in case he has an internal infection that could respond to immediate treatment.
He could be constipated, but I wouldn't wait and take a chance by treating that. Infection, if that is the cause, might be cured if caught early.
I'm not up on the best antibiotic but a broad spectrum is best, since we can't know if it is gram negative or gram positive.
I know you take care of water quality, so I would doubt it's damage from any harmful water chemistry. My hope is that it's constipation, but first... jump on treatment for infection. If his scales aren't sticking out, just has a corderoy texture to his body, it might not be too late.
Antibiotic food. He needs to eat it. Try a nuked pea for constipation, too, but go for infection immediately.
I hope he will make it.