You have to understand how council tax works. If there are two or more people living in a property, full council tax is payable unless someone is eligible for some discount. If one person is completely disregarded due to SMI, it will still depend on how many people are in the property. If the person with the disregard was the only occupant, no council tax should be payable. If there were two people, one disregard would trigger the single person discount. If there are still two people in the property after the person is disregarded - no council tax discount.
However, if your son was your husband's carer, he could be disregarded as well (this does not work for spouses I'm afraid).