Welcome to SI. You are at a very typical starting point that many of us can relate to.
It can be especially difficult when there is a child involved, and you need to know that you don't HAVE to make a decision right this second.
It takes a long time for BS's to get their wits back and think clearly about things. My suggestion is to take care of yourself and your little one as you dig for more information.
He can "insist" that his indiscretion was "just" this or that, but it's not up to him to grade the level of damage inflicted by his behavior. Unfortunately, more details and events tend to come trickling out as time progresses. There may come a point when you decide to base staying or leaving on whether he is coming completely clean and doing the work to repair your relationship.
Either way, don't pressure yourself to have all the answers right now. Check out our Healing Library for resources on how to cope and move forward, and post as often as you need for support. We're here for you.