I agree that the "grow up" comments direct at OP's DH are out of line. The way this scenario was thrust upon him is the issue, not what the scenario was.
Think of it this way. You are in a marriage where you make financial decision together. One day one spouse just shows up with a new car (even a "new" used car). The other spouse is ticked. Would the reaction be that the ticked spouse needs to get over it? Even if they were going to be needing a new car in the near future, in a relationship where financial matters are discussed, this would be a huge breach of trust. In talking it over they might very well have come to the conclusion to buy that very same car that was now sitting in the driveway, but that is all rather beside the point (though that will probably help repair the damage).
And buying a car is a *much* less big of a deal that spontaneously letting the biological father back into the son's life.
The red flags to me that indicate that the DH in this scenario is not merely acting hurt (though hurt is probably one of the feelings) are:
1. The child's memories of his biological father have been reinforced in an odd way -- memories from 2 are not that developed later in childhood without support.
2. The child has been crying about wanting to see his biological dad. Why? What is missing in his life that he's looking outside basically the only family unit he has ever known?
3. The fact that he has been crying has not been shared with his stepdad -- he has been kept out of the loop.
4. The OP's first reaction to an out of blue phone call from her ex was excitement.
5. The use of OP's phrasing "her" son rather than "our" son.
6. The first reaction of the OP was that her DH was reacting out of hurt, rather than thinking that maybe he just thought she handled things in a way that were *not* in the best interest of their son.
All of these things add up to a scenario where there is *much* more needed than for the DH to "get over" his feelings or "grow up." I think he has a real basis to fear both for the strength of his marriage and for the emotional well being of his son.
I completely agree!
This man is raising another man's child as his own, creating a home for them, all the things that the "real" "father" didn't.
I imagine how frustrating it must be for him to hear the excitement in his son's voice at talking to his "father". I am sure he feels that this man has been absent for 6 years and does not deserve to have his "homecoming" lauded like a hero. This is like if one family member always cleans and cooks and never hears a compliment, but if someone cleans or cooks once who *never* does, and gets all the accolades - except this deals with parent-child relationships, something a lot more serious.
I cannot imagine why anyone would feel that this man needs to grow up. It takes a special person to be able to raise a child that their spouse had with another person. I will be honest and say that it would be too difficult for me, personally.
He probably is struggling emotionally and mentally with this new development *and* the fact that the decision was made without his knowledge. As a PP mentioned, the OP referred to "her son" not "our son", so he might be thinking the same thing - that she doesn't consider her DH to be the real dad here when he's the one who's lived that role for all these years.
He is probably hurt and confused and questioning where he stands in his wife's eyes, and worse yet, in his son's.
I would advise the OP to hear how he feels and if she considers the boy "their son", he should have an equal voice in parenting decisions - and whatever excitement was present in their voices for the "father" should be doubled and tripled on Father's Day for the real man who stayed by their side.