Convincing him of that probably requires giving him a treaty he can present as a victory. I don't think he's going to cut his losses. Giving him what he wanted in the beginning would probably work, but US/NATO are not going to give that to him. Ukraine could do it though.
He's 69 and there's a good chance he has serious health issues. It would explain why he's taking the COVID-19 isolation so seriously.
Should do what has to be done to get him to fuck off while keeping Ukraine as a free country and mostly intact. Re-negotiate when Putin is dead.