Get them a balloon and have them blow it up as big as they can on one breath. Don't even say it's connected. It's just a game, who can blow up the balloon the biggest?
Those with poor air support are usually mouth breathing. Like, in and out only in the mouth. You can't be a mouth breather and blow up a balloon, it takes actual air support. Once they get that concept, have them apply it to horn.
Alternatively, you can also buzz the mouthpiece into the balloon.
What I did was a few times of just balloon, then just blowing through the mouthpiece into the balloon, then buzzing in to the balloon and seeing if you could get the same size all 3 ways.
How would you approach the issue of a beginner not pushing any real "volume" of air? We were doing our breathing exercises and I literally felt nothing. The students entire ability to play anything decently is crushed because of their air support.
I thought about buying one of those hospital breath volume analyzer tools to show him but I think its just an effort issue.
I think he's trying to switch to another instrument. Band is required for the next two years for him. He knows note names and fingerings well.