I am so sorry what a terrible shock to you.
I have experience in knowing the pain of infertility and then the pain of the person you went through it with hurting you so terribly, while that precious little person is so young so I truly understand.
I know you don’t want to hear the chorus of another woman, but I have NEVER known it not to be. Affairs happen within work hours more often than not. They are rarely about dates out expensive hotels, they’re more insidious, underhand and creepy than that. He has rewritten your history so he is already twisting his narrative to play the good guy who has just ‘fallen out of love’. If you watch him on his phone you’ll probably start to see the signs. A new ‘girlfriend’ will appear shortly.
All you can do right now is shore yourself up. Eat a little, keep hydrated, exercise a little and focus on being the stable parent as this unravels. He will start to lose it as the truth eeks out. Please get legal advice asap.
As for his mental health try not to go down the route of trying to save him from himself, you can suggest, you can seek support for him if your close to his family but other than that don’t get hooked on this as a possibility that could save your relationship. You need to put your own gas mask on first right now,
Again I’m so sorry, I was like you it could not be another woman, I knew him, he was incredible, honest, my best friend, he never went anywhere, I had one of the good ones, he was depressed etc etc it was an affair.
You and your precious lo deserve better than this flakey idiot!