So, the mother owns the home, but it is not her primary residence?
IMO if the son is living in the house, his mother is the owner on the deed, (even though it might have been the grandparents house at one time) if the mother is not living in the home as her primary residence, she does not need to sign recission.
Now she may need to sign some other forms since it's her property being put up for collateral!
However, I would wait for a more experienced lender to weigh on on this one!