That he now thinks he can fix.
Hmmmm.
OK, I want to apologise first that you are right in the middle of an emotional shit storm right now, and what I say might be seen as twisting the knife, making you think about what he might have done. I'm sorry - I'm not saying it tonhurt you, but because I wish my eyes had been more open when I first had my suspicions.
Mine was 'just looking, never did it' (lies) And I thought "oh he has a problem, he compartmentalises it, he doesn't want to lose me, he can change! I love him, I can support him, stand by him - that's a real marriage, right?!"
The thing is... you husband and mine had both been doing this all along. It's not something he did because you'd been together 10 years, had had a few arguments, neither of you putting the effort it, all a bit stale - then as a one off a mate egged him on drunk on a stag do in Amsterdam where it was all laid on, and seemed so far from every day life.
No. Our husbands made regular choices to fuck prostitutes at all stages of our relationships and marriages.
After I dumped him, I thought - did you do it on the night before our wedding, when we decided to be traditional and you went to stay the night apart at a local hotel? How long after our honeymoon did you do it again? And I thought - am I being dramatic?
Then... after I dumped him he got a lot more careless with technology. The evidence I'd searched for fell into my lap when he set up a new email account and had the password remember account set as one of his that we'd often shared. Yep, I was reading it because he was a lying shit that I was divorcing. I won't apologise!
As a result, I ended up with access to his adultwork account and got to see what unfolded in his next relationship.
This is what I know, bear in mind that this is a man who had already been through the consequences: divorce in progress, child involved.
What I discovered is:
- he booked a prostitute 4 weeks into his new relationship. You know, the first flush of excitement, the bit where everything is perfect and you can't keep your hands off each other.
- I later found out from his gf that she'd caught him out then taken him back because it was only once
- this bit is horrible: he took this gf and her child and ours on a family holiday. He fucked a prostitute the night before they left. And when he got back, he dropped her home, then dropped my child with me, and that night fucked another prostitute.
So that's his sorry story. I know they are not the same person. But I think it is a very useful illustration of just how deeply this "separate life" thing cuts.
If he wouldn't (or couldn't - I'm sure it's compulsive when you get used to it! I mean - sex, but done to you by a super experienced expert who makes zero demands on you and will do anything you want?! How easy is that to give up?) sorry... rambling... look, if he's always been doing this, he chose not to give it up when he was first blown away by you. When he proposed to you. When he married you. I now think I know the answer to "did my fiancé fuck a prostitute the night before our wedding?" 
If he could fuck someone else through all the best most exciting most romantic parts of your life... why is he going to stop now?
And of course, if you take him back, you have taught him that actually yes, he can get away with it.
I'm sorry to be so blunt, but using prostitutes is not a moment of head turning. It's not an affair because life not boring at home. It's a sustained decision that he can, that being with you doesn't stop that. Do not for a moment think it is as simple as him saying to himself "omg that was a close call - look what I nearly lost! - not doing that again".