I am married to a man who came 2nd, 3rd, 4th in his DMs life, always.
It took him years to be abelto articulateth hurt and damage that caused to his self esteem, sense of self worth. He is an outwardly placid man who took years to be able to get annoyed, because annoyance led to anger and anger was something he was never allowed to show... and so had never learned to deal with how it felt... or to control it. He was a bit 'light the blue touch paper', it could be scary.
Those saying your DH is behaving like a child can have absolutely no idea how harmful such an ubringing can be. If your DH has decided that he wnts to protect his DCs from similar behaviour from his parents then he is the in best place to know how far they can take it and how badly its effects are.
OP - you say that they behave in ways you don't like, so they haven't changed. Your idea of them having to learn to dela with difficult people is fine... but your DH and his feelings are the result of such behaviour... he could have a point!
And whatever you think, this is not a relationship you get to have the final say in, They are his parents.
I have said before that it is very difficult to talk to someone who has lived throgh this. Every mention of parents made my DH feel threatened, angry, out of control. There was no way of talking about them, let alone to them, without his being forced back into the silent, unloved child he used to be.
Don't force your perspective on him. He will resent it... and, if he is like mine, he will be so good, so well conditioned, at hiding that resentment, you may never know... unless he blows, as mine did! We came through it, we each had our own issues with poor parents. But I can't imagine anyone who has had ordinary, loving parents being able to really empathise and understand how deeply the hurt goes.