I am not sure if they think he is a "second class citizen" or "part of the inner circle and therefore does not need a formal invitation"....the problem is that the rest of the inner circle discusses things more often.
Bolded mine - this is an interesting point that, if true, DH would do well to remember when he feels like he is being left out on purpose.
I am hopefully thinking this is just a misunderstanding and very bad assumption about communication. Winter just mentioned above that DH's other siblings talk with the parents more often, and there was the reasonable assumption that invitations come up during those conversations. If DH is not talking with his parents as often, the subject would therefore not come up and the invitation therefore not issued.
Perhaps the parents assume the other siblings would pass the info along because why wouldn't they? And the siblings likewise assume that their parents will call DH and issue an invite because why wouldn't they?
So DH's parents may not expect him to be psychic, but perhaps they expect him to be a bit more proactive in finding out what is going on with the family since he is the odd one out by geography/time spent socializing?
Don't get me wrong, I am not taking sides and especially not siding with his parents - I think not issuing a specific invitation and then being mad when someone can't come is ridiculous.
So here we have DH on one side saying "If you wanted me to come you'd invite me (and invite me early enough)".
On the other side is his parents saying "If you wanted to come to family events you'd ask about them".
Each puts responsibility on the other, but the parents are wrong: the host is responsible, not the guest.
His parents need to realize that if they want DH there, they need to invite him instead of just assuming he will show up - or assuming that he will somehow find out about the event without them having to invite him!
This is why you should never assume...