Teaching the boring bits to young kids

My son trains at the same school as me (it's family friendly, but the kids are on one side of the hall with adults on the other). He's coming on well.

Trouble is, while he can do the stances and guard etc, he tends not to. And this is holding him back. I've noticed other young kids are the same.

So outside of class, any ideas how I can help him develop the habit of using his stances and keeping his guard up?

Some kind of game seems like the way to go, but I'm struggling to think one up.