in the normal course of things, no one has the right to consent to or decline treatment on another adult's behalf, not even next of kin (which has no basis in law anyway). Practically, docotrs usually ask next of kin what their feelings are but are not bound to follow this, it's just good practice. Usually what happens if the person is unable to consent for themselves is for doctors to make a decision based on the best interests of the patient (which could just as well be NOT giving life saving treatment as giving it)
If the person has a POA in place (there are currently 2 types - property and affairs (money) and personal welfare (health decisions)) then a person with personal welfare POA CAN make a decision as if they were the person themselves. (bearing in mind, they can only chose from the options that would have been available to the person themselves - eg they can't insist on, say, a heart transplant if that option would not have been available anyway)
As far as i know, if the husband is unable to condent or decline treatment himself because he does not have the mental capacity, he would be unable to give POA to your son anyway. The law on POA's was changed a few years ago, so i think iit will depend on whether the county council had the POA in place before then, as i think it just used to give permission for anything (not just money). If it's onlt recent, then the CC may only have the property and affairs one anyway, and not be able to make decisions about health.
This is all a long winded way of saying from the information you have given, it sounds like you son CAN'T get POA