It was wrong I know but now I know who be really is do I let on?

I found my boyfriend at the time of 1 yr on a dating site, after confronting him he said he didn't know how to get off of it and take him off using his password and username which he gave to me, so I did.
Now we've split up and been apart for around a month...
He created a new one 1 week after we split. I went on it once just see what his intentions were, I thought maybe he was just bored and wanted someone to chat too. He messaged me recently (we're now at just over a month) saying he wants me back, he still loves me and misses me!

I wouldn't have been as concerned about him dating if he hadn't asked for me back (even though a month is a very short time).
I kept an eye on what he was doing because I knew it would influence my decision on whether to give him another shot or not...

He wasn't just chatting out of boredom and has organised a date with one of them... He even lied about never being on a dating site before (on his actual profile he put he'd never been on one!) and how long it had been since we split (he told this woman it had been 3 months). I said we could talk in a month after the wedding (before he arranged the date) and he's dating people already!
I'm going to wait and see if he still messages me after his friends wedding (the main cause of out break up, which is why I said to wait until it's passed to talk).
I've made my decision now but what do I say to him? Do I let him know that I know or just say I'm moving on without letting on that I know about all of this?
It was wrong I know but now I know who be really is do I let on?
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