Anytime he does not keep his head straight in front of him and THAT is what you want him to do, just bump his mouth with the outside rein. Just do not accept his head anywhere but where you want it.
The worst thing that you accept and do not correct is the best thing you are going to get. Stop letting him control where his head is.
Again, these horses have had waaaay to much flexing done and way to little of anything else, particularly demanding forward impulsion. Good riding performance does not happen at a stand-still, especially if the horse's nose is on your foot. This is just the CA program over-done and taken to an extreme without doing the rest of the program.