I’m so sorry this is happening to you op. I know it’s hard for you to turn off your feelings atm but seriously, you have to set them aside for now and get icily practical. Trust me you will feel better once you begin to take some control over your new life instead of having things done to you.
So, although it’s a shock, try and accept that this is happening to you, put all thoughts of winning him back or reconciliation out of your mind, and tell yourself that you really do not want someone in your and your son’s life who is prepared to treat you this badly do you?
The main immediate issue here is the house:
We bought it off the council 2 years ago, so got a discount from them, so if we sell within the first 5 years we have to pay all of the discount back, so we can't do this as we will have lost out big style and neither of us have the money to buy the other one out.
Is your ex seriously suggesting that you live together for three years while he comes and goes and continues his affair elsewhere?
Sorry but that is not acceptable, as it is too confusing for your son and mental torture for you. Go and see the council (be careful what you say but find out the facts) seek out recommendations for a lawyer with knowledge of housing, or a housing charity, and get some proper advice. You cannot be the first person in this situation. And it might be better to lose out financially than to have to endure three years of hell.
Do not do anything for him in the house. Get your finances separated and sorted. Take copies of any financial documentation you can get your hands on: salary, his passport, insurance, pension, car ownership, credit cards etc etc. Secure your own finances.
Gather together as much money as you can. Do extra hours at work, get a pt job at weekend, sell clothes or any items that aren’t jointly owned, tutor or take in ironing. Anything that will help. Squirrel it away where he doesn’t have access to it.
Grey rock him. Even if it kills you, get dressed up and go out for a night with gfs minimum once a week. Tell family and friends what is happening and get some support. Try and focus on your own well-being: eating & drinking well, sleeping, and exercise. Treat this time like a boot camp. Every couple of days make sure you get out of the house for an inexpensive treat such as a hair wash and blow dry or manicure or whatever makes you feel good.
Don’t allow him to shirk his parenting responsibilities and come to an agreement about who is doing dinner/bed routine and school runs and divide evenly over the week and stick rigorously to the schedule and make sure he does as well.
Be completely factual and professional in your dealings with him. Let him know you are no push over and that you can change too. Even though you may be quaking inside, give the impression that you are independent and self-contained and controlled.
Dig deep op. Find your strength and go out and find that shit hot lawyer!