"He has been with them just around Christmas time both years but his wife has taken them during summer and for easter and he hasn't gone with them. I'm not sure why."
The obvious answer, surely, is because he doesn't have the annual leave to do so? And he trusts his wife to cope on the journey?
"It wasn't a small wedding, all her family and friends, many of his colleagues and several of his friends from the UK travelled to go. His reason for not inviting us was "We might be judgemental of his choice". He told one of his brothers but didn't invite either of them."
And right there is a big pointer, and I'm not talking about "judgemental". He didn't invite either of his brothers either. He didn't invite any of his family, although he did invite his friends. This suggests your son does not feel close to his family - any of you. There's going to be a reason for that, and you're going to have to consider what that reason is. I think lots of can see, just by reading your own words, what that reason might be, although you do seem oblivious to it.
He told you straight - that you "might be judgemental of his choice"* *. And you are! And he was able to anticipate that you would be judgemental. Have you been judgemental of his choices in the past? Answer that question honestly (to yourself at least, if not here). What was your family dynamic before he went abroad? Was he the Favourite Son, or the Criticised Son, or the Disappointing Son, or - what? Were you in the habit of comparing your sons, and finding him wanting when you did so? How did he feel about his place within the family before he went?
"We see both of our other sons most days, we provide childcare for our other grandchildren and have them all for dinner on Wednesdays. They don't see much of their brother either."
Frankly, that statement suggests several possibilities. But given your son has chosen to largely absent himself from his parents AND his brothers, it's more likely that you are all rather unhealthily enmeshed than that you are a big happy Waltons-like extended family. Do your other two sons/wives/DCs all come on Wednesday to ensure the continuance of free childcare, or because this is how they want to spend their lives? Who suggested this Weekly Gathering?
I think you need to take a step back and look at how your family functions, and - more importantly - how it functioned before he went abroad to work.
As an aside, and this is about me and not you, I would be a bit judgemental about your son's relationship. But that's because I would be looking askance at a 31-year-old man who marries a 21-year-old woman, not the other way around. It bespeaks a desire to be the more powerful in an asymmetric relationship, and that's never going to be good for the younger woman. And again, I'd be wondering where that desire to be powerful came from, possibly from feeling powerless in his youth. Any ideas how he might feel that way?