Faceing the same thing with my son at age 14 I wrote my ex and told her that he would no longer be required to accompany his younger brother when I appeared to pick up the younger one for weekend visitation.

I then stopped all communication with the older boy. No holiday/birthday cards even. Communication was zero. Ten years later the older boy sent a message saying he wanted to re-establish contact and I agreed. Although I don't like the man this boy turned out to be (exactly like his mother and step-father) I respond when contacted and treat him with respect as if he was equal in my eyes to his younger brother.

With a ex actively working on (in my case) a Mama's Boy there was no point in beating my head against the wall and experiencing the constant rejection. Funny thing with the younger boy he never bought into his mothers lies and later when he was an adult I asked him why is response was a simple "it was more fun being with you".

I know there is more to it as the older Mama's boy was noticeably the favorite of their mother and by then the step-father had run out of credibility by never following through on his promises of doing things with and for the kids.