Is my son being unreasonable?

I wouldn't say anything if I were you Lynda I'm sure if your son wants to discuss it with his brother he will do.

Don't worry about it, just enjoy the wonderful day when it arrives.

Does the eldest son have to have his brother as best man. Does he not have a close friend?

I faced a similar problem when I married. I have two sisters and as I was not having a big wedding, just needed a 'Best Woman', not brides maids. I felt it was invidious to choose one and not the other so I decided to ask my closest friend to fill the role.

I discussed it with both sisters and they were both happy with the decision.

Maybe the youngest appears the less busy then - who knows

Weddings, who'd have 'em!

Usually nothing good can come from getting involved in wedding angst. Particularly seeing as in most (not all before anyone jumps on me) cases these decisions are made by the bride not the groom.
That said, don't think there's any harm in casually saying 'oh, so what will * (middle son) be doing for the wedding?" If he say, oh he'll be too busy then you could say you are sure hed make the time for something so important but leave it at that. Good luck - sure it will be a lovely day regardless how all this plays out.

It depends on your relationship with your sons. I would talk to the middle son and ask how he really felt. If he says fine. Fine. If not I would talk to my eldest son and ask whether he could have two best men? The last wedding I went to had two best men and it worked well.
But I also kind of agree with thatbags. Adults are often best at sorting stuff out themselves. I am such a fence sitter!

He's got a girlfriend and a young son.
Why do wedding always cause such potential pitfalls...?

Assuming the youngest is single and the middle one is 'very busy with his own family' I don't think your son is being at all unreasonable!

Again assuming, the middle brother will be invited to the wedding so I can't see the problem.

I'd say nothing.

I'd keep out of it if I were you. They are adults. Leave them to it.

Is the youngest son single?