All I can say is: do not sell up. Do not do it.
You will put yourself in an awful situation, not a homeowner and at the mercy of the rental market.
Really, don't do it. It's the last resort and I mean really hang on and do everything not to sell up.
You don't owe him anything. He's lived in your house and his cost for living somewhere has been £425 a month then £600 for two years or more, which is extremely low and not covering his actual cost at all, then he has paid £1,000 plus £400 bills for the past 20 months. BUT, that higher payment only really offsets the under payment for the previous 2+ years. He doesn't get back the money he paid out to live monthly in your house. We all have to pay out probably a minimum of around £1400 a month just to have a home and pay bills, if we're lucky.
His association with you has, by the sound of it, allowed him to pay of substantial debts and he's also been able to use a gift of £20k for his own holiday and not shared it.
You owe him nothing. He's been nice to your kids but now he's ending your relationship and demanding so much money from their mum that they will lose their home. So you have to be really TOUGH here. This is super serious.
Say to him:
I'm sorry but what you paid was just a normal contribution to monthly accommodation and bills, if we spread it from 2021 when you moved in. So I don't believe I owe you anything. Goodbye.
His only option will be to pay to take you to court to demand the money and... he will not get it. He knows that. That's why he's trying to pressurise you now.
This happened to me but in a much worse way and far far more money. And mine and my child's life have been devastated by my selling my house.
So I am telling you now: DO NOT SELL YOUR HOUSE. DO NOT GIVE HIM ANY MONEY. DON'T FEEL GUILTY. IT'S FINE, HE DID OK.
and that is it.
Please please think of your kids. They are way more important than him. And he is debt free and ok and was financially secure enough to blow 20k on a holiday that didn't include you so quite frankly - no. Let him bugger off.
Think of your kids in rented accommodation then after a few months you have to move. Then again. Then again. Think of them crying about it. Think of all of that.
Don't risk it.
And be aware: he could move on quickly with someone else. Do you want to sell your house so he can spend your money on someone else, while your kids lose their home?
You have the right just to say no.