oh suzi - i feel for you. our kids are so fickle, and the saying that ' when they are little they break your arms, when they are bigger they break your hearts' is so true at times.
i had a similar experience in that when my son was growing up his dad, who left us when my son was 7, could not be bothered at all. He never saw him, never took him out, never sent a birthday or xmas card etc. when i tried to tell him that he would regret what he was missing, he simply told me in an arrogant way that his son would always want him.
Lo and behold, as soon as he is old enough to be 'useful' - he wants him back in his life and there is nothing i can do about it.
It is painful, but i am confident that he knows that i love him enough to stand back and let him do what makes him happy, as long as he is safe.
i know that deep down hw will always remember who was there for him in his times of need, and who will always be there for him no matter what.
At 16 your son is still immature, vulnerable and probably gullible. Children are fiercely loyal to their parents - his dad is probably his hero now. Maturity will help him see things clearer one day. In the meantime, its hard going.
Just be there for him, dont play into your ex's hands.
sorry i cant be of any help.
xx