((HUGS)) for both you and DD.
For what it's worth, I believe him that it's not that he didn't love you or want to be with you, but when it was offered on a plate as a release from the stress of life with DS he took it. He's weak, but many people are. Weak doesn't necessarily mean inherently bad either.
I also think he's telling you the truth when he says if they hadn't been caught it would still be going on, but that being caught pulled him up short and made him realise what he was actually doing and what he was risking.
You are doing the right thing not rushing into any decisions. You are also being very sensible when you say that you don't need to hear him talking to her and that even if you do, there's nothing to stop him calling her again when you aren't around.
There is no point at all in policing his movements, monitoring his phone, laying down the law about things - because there is only any point in him being there if he wants to be there.
Tell him what your boundaries are and that if he steps over them it will be the end of your relationship. Tell him it's not a threat, it's a promise.
While he isn't living there, please make sure he is doing at least his fair share of looking after the kids. He needs to spend time looking after DS so that you can go out and start rebuilding yourself. It is hard with kids, it's bloody hard with a child with SN. He needs to give you the time to be YOU.
I know what you mean about understanding him being tempted as she's young, lovely & beautiful, while you are 40, overweight and stressed :( It's time for you to make him step up to the plate with DS so that you can find YOU again, so that you can feel lovely & beautiful again and not just frumpy stressed Mum.
He needs to rebuild his relationship with your DD. He needs to spend time with her and talk to her. He needs to explain why/how it happened and why he wasn't strong enough to resist it. He needs to tell her how he would resist it in future if the same thing happened. He needs to tell her that no woman ever needs to put up with this and how unbelievably fortunate he would be if you were to forgive him and take him back. He needs to tell her that she did the right thing telling you and that he is the only one to blame. He needs to treat her like the teenager she almost is and not like a small child. This will have a really huge impact on her life - not just now with Mum & Dad, but in her future relationships and he cannot be allowed to weasel out of dealing with this impact he has had on her life. Sweeping it under the mat is not an option.
I am sure you can get through this together... so long as you both want it and it sounds like you do.