His home is much nicer than mine. He is rubbish with money, spends everything he earns and is in a lot of debt. He doesn't mind being in debt.
Re-read your description of him re money and then what you have written about your attitude to money and debt.
There are absolutely no guarantees that you won't end up with shared debt. You will lose your independence and lose the control you currently have over your own finances. How can you (or he, for that matter) guarantee he would pay the mortgage?
The arrangement he is proposing requires you to sell your safety net and put money into a property for him. Your property may not be as flashy as his but it is all yours.
I would want a watertight legal financial agreement in place to protect my assets if I were in your position... but you have not had anything positive to say about his attitude to money and debt and I would suggest that you are incompatible on that front.
You have no guarantee either, that you could work part-time nor that he would step up to ensure you had a good pension.
I understand why what he is proposing might sound a reasonable proposition but he has far less to lose than you do - it would be beneficial to him.
Just think - he is renting the property he aspires to, wants to own it but all the jeopardy is on you. He didn't find a home he could afford? You say he is well-paid? So what? He is not a safe bet.
You and he may be getting on better but that could be because you live separately (and maybe money and debt were issues in your marriage; issues which you don't have now you live apart).
Or it could be because he's playing the long game and presenting himself in the best possible light to make you believe you could live together again in order to get his hands on your financial asset!
How would that be a better way to live than when you lived together before? I think you would find being in such close day in day out proximity to him AND financially tethered to him along with his 'casual' attitude to money would not the safe haven of security he is describing or that you would hope for.
I understand you'd like to retire, have time to help your family and a better pension - it just sounds so risky...