A man with a genetic disease marries a woman who does not carry the disease. It is not possible for their son to carry the disease. The disease must be?

The disease must be an X-linked disease (both dominant and recessive are possible), or a recessive trait.

The son will inherit one of his mom's X chromosomes and his dad's Y chromosome. If it is impossible for the son to get it, it certainly is not located on the Y chromosome, which he will get from his dad.

It could be on his dad's X chromosome, either dominant or recessive, and he would have a 0% chance of getting it, since he will not get his dad's X chromosome.

It could also just be a recessive, non sex-linked trait, in which case he could not possibly get it because his mom is not a carrier.