It’s up to you to decide what matters the most for you:
The age gap which seems to be bothering you is kind of a moral dilemma (legally, they’re both allowed to do what they want), am I right? So, does your principles matter more than having an opportunity to spend some time with your son and maybe get to know his boyfriend a little more than by its age? If that can help you, it’s not that rare. Here in France, our very own President (Emmanuel Macron) is some 30 years younger than his spouse (something like that, I have not checked).
Have you privately discussed what bothers you with your son? What was is opinion on the question? Do you think his judgment was blinded in anyway, or is it just you that feel not at ease with a 20+ years age gap?
Can you be OK with not liking someone (that has done nothing wrong) but still be sincerely welcoming to them knowing it will make your son happy?
Do you want to stay a relevant part of you son’s future live? I mean, your son is now an adult and will move on to build his own live with his own family, with or without you being part of it and that’s up to you. It could be with that older dude or with someone else (maybe even older, or younger? it’s up to them) at 21 it’s hard to say. But he will live the live he wants, and that will include many mistakes, whether you want him or not. We all have done mistakes, some more often and some more serious than others but well have done some, I’m willing to bet you did some to right?
Is that King dude an asshole of some sort or is it just you you not liking him being older?
If you’re sure he is doing some huge mistake, do you want to still be there for him when things will turn bad? No need to lie to your son and pretend you’re happy if you’re not (talk to him, openly but not in a judgmental way it’s all that should matter. Just share your opinion/worries and let him understand that no matter what you’re OK with whatever he decides to do).
BTW, there is no right or wrong answer here (I know what I would wish to answer, but that’s just me). It is the kind of questions I would ask myself if I was facing similar doubts as you are. Hope this helps.
It’s up to you to decide what matters the most for you:
BTW, there is no right or wrong answer here (I know what I would wish to answer, but that’s just me). It is the kind of questions I would ask myself if I was facing similar doubts as you are. Hope this helps.