Legalities aside, do you really feel like your grandson is at risk for abduction? Because if neither of you do, and you are seriously considering reporting the mum, I don’t think either of you can truly love your grandson. If you really don’t believe your grandson is at risk, then the very thought of destabilizing his home life is shameful and evil.
Given the history, your son is at best a part time father. Should the mother have informed with detail about the holiday? Absolutely! Should she have made time for the facetime? Of course! But you both knew she was taking the child on holiday. You could have asked questions like adults. She wasn’t trying to hide it.
If I am wrong and this child is at risk for abduction, let me apologize in advance. But if you are pondering this spiteful pettiness because your son’s ego was bruised and you couldn’t “send the grandson something”, please do some soul searching before you attempt to make contact with this child again. He isn’t pawn for revenge. Please do not treat his life as a vehicle for payback.