Is he somebody that generally does not like waste? How annoying, it seems that he makes it your fault because you do not want his leftovers.
I would tell him to plate up his own food, what ever he serves himself is his.
I would also talk to him about it when he is not eating, like you do with children, telling him what you expect of him and how much it annoys you.
I've got another 'keen'... Ouch!
Angela Rayner lashes out and calls Sunak “pint sized loser”.