Men seem to have the ability to simply, switch off, slice through all the bonds and skip away leaving devastation.
True colours often don't show until pregnancy or early childhood.
It's possible that he can't accept that after all those years of being the centre of your universe, another (male) person has come along and displaced him for your attention.
And unlike you, he additionally has no visceral connection with your son.
You MUST get legal advice. Do not let him tell you what's going to happen, what you're going to do, what the arrangements are, what rights you have. Do not let him tell you 'no need for a solicitor we can arrange it all ourselves.'
You and your child have RIGHTS.
Find a solicitor who will give a free consultation to orientate you.
And any subsequent fees must come from your joint finances.
Remember you co own everything.
Channel your shock and grief to anger and resolve and get a full understanding of the financial situation and assets. His bank statements etc too.
Because he has been thinking about this for a long time and has already been making his plans, leaving you behind and in the dark. Catch up. Overtake him.
I would also warn you to be prepared for someone else on the scene very quickly, despite assurances or appearances.
Tell your line manager. You may need compassionate leave, and might even be able to access legal advice through any employee assistance programme you may have.
All the best to you and your child.