I can only give advice from my own pov.
When I went back to work after my first child, I threw myself into work. Including the social scene.
Deep down I felt more pressure to secure my job. In my industry the nights out are a big deal. People who attend do go further. It's shit but it's a fact.
I wasn't the sole earner, but I was so scared of losing my job I really tried my hardest. Eventually I had enough and chilled and realised being good at my job was enough.
But for a while I did think if my bosses liked me more, if I was more sociable, put extra effort in...I was safe.
Christmas is the time of year that business out extra pressure on their staff to be sociable. It's really difficult to try and balance it.
What I am saying is he may be giving into more pressure because he now is responsible for a baby.
I would speak to him and tell him how you feel and ask him how he feels about it. No accusations, just a calm discussion and go from there.
This is all based on the fact that you said he is a good man. The other alternative could be that he is being selfish and grabbing his freedom and shirking responsibility whilst using work as an excuse.
In which case a serious chat is needed, because this isn't ok.