Simon, I can't imagine how it must feel to have had not contact with your son. I know it was his choice to stay with his mother but at his age I would've thought he would still stay in contact BUT then we none of us can really say what another will do even when they are our flesh and blood.
I hope this situation improves soon.
Me 43 XH 45 M 2.7.88 Divorce 7.10.09 Kids D20,S17 & D15