I think there are two separate issues here :
1) The Son's situation
2) The father's situation
These should be dealt with separately.
The son is like a lot of other people. For the time being he is stuck and cannot pay his mortgage. I think its a bit drastic to start running out to sell the house. The amounts of money involved are not huge and presumably the son needs somewhere to live. The son needs to come to an agreement with the bank, probably along the lines of a split mortgage to pay something small each month until his situation improves. If he cannot make any progress with the bank informally then I would encourage him to apply for a PIA.
In my view the father has nothing to worry about. He signed up as a guarantor but is insolvent and already has a judgment mortgage of 500K on his share of the house. The bank may well have a look at his situation but I very much doubt that it will take any action against him, given his level of indebtedness