I think the building society and the solicitor is right. When both parents are dead the building society need to sell the house.
If there is a sitting tenant - the son - it will be extremely difficult. They may have to take him to court to evict him and if his solicitor can show he did not know what he signed, or understand, then the building society would not have a leg to stand on.
With retirement plan mortgages the lender does not get the capital until the house is sold on the 2nd death.
It is not discrimination - it is the law. It is intended to protect the mentally disabled from signing documents they cannot possibly understand that would be to their detriment.
Can you imagine the outcry if the lender is able to evict him when he has just lost his parents, so the house can be sold?
As I said, the building society and solicitor are both in the right in this instance.