First, welcome to SI...I'm really glad you found us just as I am really sorry you had reason to need us. However, you won't find a better group of people and you are no longer alone dealing with this.
Wow. From what you describe, it sounds like --at best-- your H has extremely poor boundaries and a need for a lot of ego stroking. Maybe I'm having a moment's struggle with transference issues...but it sounds like you found the tip of the iceberg.
My H didn't initially 'confess' to anything I couldn't nail him to the floor on with details I knew....I don't think he was particularly special in that regard, it's just he had a LONG history of infidelity so his lies of omission were quite substantial.
Your H is a broken person. A healthy married person doesn't make out with a stranger in the shadow of a carseat...or respond to Craigslist personal ads...or exhibit any of the other special cheater behaviors. Is your H seeking some individual therapy to dig out why he felt entitled to abandon his vows to you and needed so much external validation?
At this point, due to his massive lies of omission, I would only trust what you can verify, and assume there may be much more than what he's confessed to. You don't mention how long you've been together; you did note your child's young. Is music his full-time profession or something he does in addition to another line of work? (The reason I ask is not to snoop but rather that IMHO, there are professions and/or working situations that can make R especially difficult.)
I can still remember how I felt when I first posted what I'd discovered on DDay 1: namely, that my FWH had a secret "mySpace" account with one friend, a female. Feeling a little defensive when I read replies warning that there might be a lot more to the story...but I'm glad it put that thought in my head, because if you read my profile you'll see that oh boy were they ever right!
I wouldn't trust that he just made out with her...that text implies more than a fleeting casual encounter. I'd definitely think about STD testing. I wouldn't reveal what you've learned from the history if you haven't already; I'd try to find out as much as I could before....others may have differing opinions on this.
Two weeks is a very short period of time in the R process.
I highly recommend getting a copy of the book, "Not Just Friends" by Shirley Glass. Excellent reading, my hands-down favorite book on infidelity.