Its tragic that your son abused or neglected the bike in such a way... really too bad, for both of them.
Honestly, looking at the pictures, that appears to be too nice a bike to consider parting out. If it were me, I would replace the engine with a good, used one. After that, Keep it for the other family members or yourself to ride, or if its not needed any longer, sell it to someone who will APPRECIATE the bike.
As for your son, I think now is a good time to set down the "you save up and buy your own bike" rule... that way if he abuses or neglects it, it's his own time, effort, and money that is wasted.