Get a canvas and make it into a sack shape, fill with sand (weights on top to settle it down, obviously remove before training lol), duck tape into shape and add foam and more tape to keep it in place around the solid bottom (depending on his kick height). If you really want to be mental, tape bamboo sticks around instead of foam and make sure you put a lot of tape evenly around the edges
When he kicks it, it will hurt mate but before you know it he’ll have shins like titanium ?
Never let him walk away without feeling like he did one more kick when he thought he couldn’t