It’s going to cost you to move, estate agent fees, removals, solicitor fees, stamp duty (maybe)
Around £10,000 on your type of property.
Borrow that £15,000 against the equity and get people in to rewrite and plaster board the house and give the kids a bed room.
Listen to @BluebellsGreenbells. He's running away from a problem of his own creation. And it's not just him, it's you and your DC along for the ride.
There's a lot to be said for realising that a situation isn't helping your MH and letting go, but not exploring other solutions which could save you literally tens of thousands of pounds in the long run is criminal. That's all family money he's willing to wave goodbye to because he's mentally checked out of your current home.
We're in the middle of a house sale and purchase atm and it's fucking stressful. Skipping out on your house, marketing it in its current state, trying to find a new property, negotiating the sale and purchase, dealing with a chain....it won't be any less stressful, and in the end you'll be £££ out of pocket and still in a (new) house that according to him will need work!
Set a budget of 10-15k which as Bluebell says would otherwise be spent on moving costs, and get someone in to get past this stage where he's stalled. Once there's some meaningful progress and he's not looking at the same to-do list he may well find he's able to see it as a new project and start afresh.