Do you have a coach? If not, get one even for a couple of lessons. This isn’t due to a nagging leg or other things, it’s the rider not having the skills to address the fact they’re letting the horse get away with things. You’re always training a horse or untraining it. So basically, MAKE him listen to your leg and don’t take no for an answer. Expect him to balk at first because he’s gotten away with it and won’t understand why he still can’t.