It is difficult to say, especially without more information.
He could be a rex. Rexes have curly/kinky/wavy hair though, not thin haircoats. I hear rexes can sometimes oscillate between poor and good hair coats as they mature.
Since the mother was normal coat it can't be a double rex, which often have thin hair. Are you SURE the mother was not just a poor rex? They can sometimes appear almost smooth coated but often bushier or slightly wavy, and will always keep their kinky/curly whiskers.
He could be a hairless, as they can grow patchy, thin fur and then loose it. Some breeders are refusing to breed true hairless rats because of their inherent problems.
How do you know the mother but not the father?
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NMGA miniature meat goats, Silver Fox and NZW Rabbits, Muscovy ducks, several breeds of chickens, fancy mice and rats in Michigan