Have you decided if you want to stay with him after this confession?
Why did he tell you about all this do you think? To absolve himself of guilt (selfish amends) or because he thought you were going to find out anyway (from your 'friend'? Just to clarify, in AA no good sponsor would suggest making a confession like this, which causes someone else extreme pain. Is he actually working a program of recovery? Being sober is not the same as being in recovery you know.
Have you considered AlAnon for your self? He may not be drinking, but sounds like he still has an addictive mindset and behaviours, so it would be worth reaching out for help with boundary setting for your own sanity (whether you stay with him or not, as he'll still be in your life as babies father regardless).
BB