The first is whether the house is registered in the name of the parents, and the second is whether the parents agree and sign when the house is mortgaged.

If it can be proved that the house was mortgaged by the son with a forged signature without the parents' knowledge, it can be proved that the mortgage is invalid, or the mortgage is naturally invalid after the loan is paid off.

You can negotiate with your son first and pay off the loan, so you don't have to sue him. If your son forcibly occupies his parents' house, then your parents have no place to live, and you can sue in court.