ESH.
yes he is your son but he is also your ex's son, and she is in his life a lot more than you, plus she grew him in her body. you call having him at weekends and showing up for his birth 'being a very hands on dad', but i would call it 'the bare minimum'.
i do think that dropping your ex's old name and double barreling it with her new fiance's name is a good idea though and that does make sense. it's also very rude of them to not even consider your opinion and just want to push trough.
i would say have your son have a combined name of you and your ex, and your ex's future children could get a combined name of your ex and her wife? or let the kid choose which name he wants.