I think it depends on whether your husband's sadness is ruining these events for your son. I am not quite clear on whether that's the case, or whether it's simply upsetting you that you realise he is sad at these times.
If it's the former, then yes he needs to get his act together for the little one's sake, and fake it till he makes it if necessary. He has a responsibility to make the little one feel as loved, secure and happy as he can.
But if he is simply revealing the sadness to you, then I think you have to suck that up and support him as much as you can. Estrangement from one's child must be utterly unbearable - I can't imagine the constant pain he must feel about this. Surely, now you are a mother, you can see that.
In fact, I am surprised that the main locus of your discontent seems to be about your son's "milestones'. Surely the much more urgent issue you are facing, is that the man you love has lost contact with his eldest child. That's the big deal here, that's what you both need to address.
He'd be a monstrously unfeeling person if he WASN'T devastated about never seeing his son. And surely you don't want to be married to a monster.
If I were you I would do everything in my power to help my husband salvage his relationship with his older son - simply having "reached out" is not enough - unless he has been explicitly asked not to I hope he makes regular contact with his first son, telling him how much he loves him, apologising for hurting him, making it clear that that son is deeply missed and would be welcome as a part of his life in whatever way the son chose.
He should regularly offer to visit him, or to book a trip for them both, or whatever contact might prove acceptanle.
As a first step I would sit down with your husband tonight, when the little one is in bed, and have a full and frank conversation in which you are totally honest with yourselves and each other - is there any more your husband can do to rebuild that fractured relationship..... The do it, whatever it takes.
You can't ever expect your man to be happy while half his heart is broken. Your role should be to support and encourage him to do everything he can to heal the rift, so that he doesn't lose any more precious years......