Poor little lad. There is nothing wrapped around the penis is there like a hair or something? This can prevent the blood draining from the penis and it becomes swollen and then cannot be retracted. You are right to keep replacing it as it can get damaged and sore if it is not inside the sheath. I think it is worth trying antibiotics. He may have some congenital abnormality but I would have expected this to show up when he was younger. There is some info here and your vet sounds to have done what is suggested.
https://www.msdvetmanual.com/reprodu...-small-animals.
It may be worth taking the wheel out for a few days just in case the penis is getting rubbed as he runs and swelling then prolapsing again.
I hope he does get better soon,