I own a property abroad and we just spent a holiday there together and he knows my parents, my parents house and whom I inherited from (My granddad was a famous German architect) it is easy to google what I inherited and how much the assets are worth.
Your wealth is real. His sounds like it is all smoke and mirrors.
Really sounds like a horrible time, and it must be a really hard time for you with your mum being ill. But to be honest, however much he has supported you, he sounds rather shallow and materialistic (the flash watches, the PR parties, etc).
This is a really impertinent and may seem rather a cruel question, but do you think he would still want to marry you if you had nothing, no family wealth? I think you need to ask yourself.
You don't want your property being used as security against his business deals in the future. He could lose you everything.
And I know it sounds cruel, but if he's a young man and he's accrued 280k of debt, he's not just overly extravagant, he's not a very good businessman either.