I suspect it’s cash he’s not declared as part of his tax return that he’s trying to convert into something legit?
Legally you’re liable if you assist or suspect the cash is illegal, & if he’s doing it to avoid stamp duty then it becomes a criminal charge that you would be assisting in, so you become part of it.
Given your house sale (if it’s in your name) is tax free, not sure where the upside is for you (x actually selling your house), so as long as you tell him you’ll be declaring the cash & paying it into your bank you’re completely in the clear……though I suspect that will change things for him