Help him to buy his own place if you have some cash or can remortgage.
A “loan” of £10k should kickstart a deposit. Put in a repayment plan but don’t expect it back. It might be more than you would have got back in “rent” but it will help him stand on his own two feet. Use your liquidity you got from gains your generation made to help him.
He should look to get a lodger, that should pay a fair chunk of your loan back. Rent a room is tax free inc extra services.
My parents did this for me. They remortgaged their house interest only a little to lend me a deposit, and I paid both their mortgage and my own with the help of lodgers.
House prices will only ever go one way and the sooner he has his own place the better.