He has to take care of his own mental health and sort himself out independently. From his post, he has been tried to force himself to be happy for a long time.. It's become too much. He doesn't want to pretend any longer..
Clearly nobody can force happiness on themselves.. He has to primarily become the source of his own happiness, and then he can add extensions to his happiness. ..such as family, wife, kids, friends, etc... He has to find himself and happiness away from the current dynamic.
I'm not understanding why you felt the two of you were happy, especially since you've known about him battling depression for awhile it seems...even before deciding to bring an innocent human being into the mix.
Depression is a lifelong battle. It's not something curable. Life's unpredictabilities (a lot of them very traumatizing and negatively life altering) will always happen.
It's probably best to respect his feelings, perhaps divorce, perhaps work towards moving on in therapy.
If you didn't know beforehand what type of person you were marrying, you know now. He is never going to be the person, the partner, the husband, that you need and want. He is never going to be the father that his son needs and wants.
That's just what it is.
He has been fighting and losing a battle that he can't seem to win.
Whether it's someone else or not and truly his depression...
He's unhappy, unhealthy, and wants out.
Let him go. It's not good if he stays.
I'm sorry