All I can do is just confirm what everyone on here states.
The child, does not cease to be your g.son, just because his parents have broken up. You are still, and will always be, his g.mother. Poor chap - having to deal with all the trauma of Mummy and Daddy not being together. Bad enough for him.
No-one, at any time, under any circumstances, should try to use their child/children as pawns because adults break up.
Obviously, you should be sending love, kisses, card and present to this child. AND.....persuade your son to do likewise.
Not very often that GSM and I agree on anything - perhaps the fact that we do, absolutely, on this matter, says it all.