Tap him with a sip or start lunging him and do walk trot walk trot walk trot Canter trot walk trot
Sounds like he needs to listen to your aids and not anticipate what's next, so don't let him anticipate! Whenever a horse starts to develop a habit like that, I try to change up my routine. I like the other suggestions of using sound or other aids (i.e. smacking your boot to make a sound, or waving a whip so he can see it), but have you tried using poles at the walk and trot, and working on a ton of transitions? Trot to walk, trot to halt, walk to trot, etc. during your warm up? Change direction a lot, do serpentines, practice changing your bend if you can. If he stops and won't move, can you make him back up until he's back under himself, then ask him to go forward? Basically, I would suggest trying a lot of different transitions to get him focused on you and not what he's used to doing. Good luck!
Try to get somebody to use lunge whip and hit it on the ground so it makes a noise, or I've found that using a dressage whip and waving it is really helpful!
Maybe trying taking a crop a hitting on your boot. The sound should make him move if he's not spooky.