Specifically from a mortgage perspective, it might be easier for him to take out a mortgage for 160k to buy the house from you, and you can gift him the money afterwards. Otherwise he's effectively taking out an equity release for the works, which there's less appetite for among the banks, but it's possible. They might treat similar to a self build mortgage, and look for engineer sign offs for stage payment, etc. He could end up having to borrow at home improvement rates over a shorter term if he can't get mortgage.
Normal central bank limits of 3.5 times income would apply, and he'd need to evidence repayment capacity.