I wouldn't worry about explaining anything to your son. I left my x when my youngest was 14 months old, and I don't think he was affected negatively. YOur child has the stability of still being in the same house.
PLUS, I think an 18 month old's memory is only about a month - 6 wks, not sure, so what is still raw and torture for you, is already the new normal for him.
Good for you for telling your x that he needs to respect the pain HE has caused you. He pulled the rug from under you, and although of course he is entitled to continue to see his son, I think he has forfeited the right to saunter in and out of your home, your space. HE MUST allow you to heal a bit, toughen up, get STRONGER. YOu can do that. You will do that, but it'll be easier if you have a bit of space.
Please talk to a friend and to your Dad today.
I really feel for you, because the rejction that I mentioned in my post ystrdy was a long time ago now, but I felt beyond devastated. Totally deconstructed. When I got better, I felt 're-built'! A lot stronger, more clear of my personality, but I more empathy too. I hope that gives you hope that you will really bounce back with bells on. You will. I can't imagine going through that kind of pain AND having to look after a child/children.
Take care, and come back here and chat. You can vent about everything, and have a laugh too. On mumsnet there's ALWAYS somebody in the same boat, even though in RL you can feel you are the only one who isn't happily married!