I agree with the others, counseling together and individually for yourself as well if you can swing it.
I’m assuming your separation didn’t span years, how long were these two together? Maybe 6 months? In that case, it’s important to remember that what you’re reading is the honeymoon phase of their relationship. The period where everything is new and the sex is so good and you just think about that person all the time. There’s no late nights with a new baby, or worries about money because someone lost their job, or having to pick her hair out of the drain for the millionth fucking time. You can’t compare a relationship nearly 2 decades in to that.
The fact that he lied about her meeting your child is a big deal. That’s a betrayal of your parenting relationship and a serious reason not to trust him. Depending on the age of your child that might also mean he instructed your kid to lie to you, and if that’s the case I’m not sure you can, or should, come back from that.
Assuming that’s not the case and you want to try to salvage things, I think the really important takeaway is: you now know he’s capable of the emotional intimacy you’ve been wanting, but he’s still not giving it you. You say you worry he got back together with you out of convenience, but why did you get back together with him? Did he commit to giving you what you needed and that’s not happening? Or did you simply hope he’d changed? Make a list of the things you need, the timeline you need them in, and how he might address your very valid concerns around lying and trust. Also write down your plans if these things aren’t addressed – is it to continue with the divorce? Then take that list to couples therapy.