Shaz what you are learning from this horrible experience is that people having affairs lie continuously. They lie to the affair partner, their spouse and very often, to themselves.
As you know, I challenged this nonsense that he had gone off you first before starting this affair. It so rarely happens, but like so many before him and after him, he is re-writing history, to make himself feel better. No doubt he is telling OW the same hackneyed lines, because like him, she needs a justification for her behaviour too.
Of course he has slept with her. No man leaves his wife and family for a kiss. This started as an emotional affair and as soon as he had sex, the feelings appeared to deepen, because he is someone who cannot divorce sex from love.
I suspect he had sex with her for the first time around April, because that's when he's telling you this started, but what he means is that that's when he was first sexually unfaithful. Up till that point, he could kid himself that since he hadn't had sex, this wasn't an affair. The phone bills will tell you when this really started, in terms of the emotional infidelity.
He is comparing the heady flush of a new relationship with the more settled depth of a longstanding relationship. I'll bet he felt like he does now for you once, too.
He is in the grip of a form of insanity at the moment, but like everything, this too will pass.
What usually intrudes on it and brings it to an end is a hard dose of real-life, recrimination and loss. Because they have both been in the grip of a "this thing is bigger than both of us" fantasy, they haven't troubled themselves with real world consequences. The disgust from others at work, the horror of their families, the OW's friends who are now re-thinking whether they could ever be friends with a woman who has an affair with a young father and conspires in him leaving. The prospect of having no money and a life of step-parenthood. The prospect of another man being a day-to-day father to his son, in time. The image of you having sex with another man.
All of a sudden, this great romance looks more and more like just another tawdry affair.
Information is power Shaz, in all its forms. The worst is actually over, because that was when you were being deceived but had no proof.
Get as much information as you can about his relationship with her and your legal and financial position. Stay firm in telling him that it is over as far as you are concerned. Tell him perhaps that you now know he has been sexually unfaithful and can prove it but you will not be telling him how you've got that proof. That should cause them a few sleepless nights in that hotel.