I'm very sorry op, did you see this coming, had he checked out previously were there signs looking back?
I agree , he has had his head turned, whether that will work out for him in the long run, who knows but for now you have to protect yourself.
He has just hit you with this like a bat and you must be reeling. He is not upset, he may feel some guilt but not enough to stop him being incredibly selfish and abandoning his family.
Getting your finanacials sorted will bring him back to earth.
Splitting assets, putting in place childcare arrangements and child maintenance will make him feel less like a single man.
You have been let onto the secret now, although he won't admit it, you now must act to protect yourself and your child.
Please make an appointment with a solicitor as soon as possible, ask for outside help with family and friends, explain your situation, let them help you.
His lies are now going to be visable to the outside world, this could make him very unpredictable, if he becomes aggresive phone the police.
Take care and look after yourself as much as possible.
Sendings hugs.