I would work on some exercises to get his head softer but I would not recommend using a stronger bit
Try riding bareback about 1-2 a week it will help with your bond. Also try join up (run him/her around until he/she shows the inside to you, lowers the head, and licks the lips!!) works and he/she will start to listen
Try vocal cues. Some horses were trained vocally or even message the previous owner for advice since she/he would know the horse well and can help you out.
First have his teeth checked. If that's not the problem and stronger bit is not going to help. Go back to slow work. I work most of mine a just a snaffle. I get them really soft ad responsive at the walk and trot before I even touch the lope. Flexing at the poll and building muscle in the hindquarters would also be helpful.
"Speak softly and carry a big stick" ....ask gently first by saying woah and sitting back, then pull him into a sharp ones rein stop if he does not respond.
If he is headstrong, getting a stronger bit isn't a problem. But you want to make sure that you are more gentle in his mouth. If you are gentle in his mouth then he will be gentle in your hands. When trying to get him to stop, sink your heels deep in your stirrups and sit deep in your saddle and half halt him into your downward transition. Id that doesn't work you could bring him into a circle if he won't stop, and gradually make that circle smaller until he does. Also working on your trot-canter and canter-trot transitions will help.
If it turns out to not be a tooth problem, I could suggest a kineton which works wonders for me when I ride my gelding at a drag-hunt and pulls like crazy
Try cantering in a circle and make it so small he has to stop. Once it gets that small, tell him to stop. Horses are lazy animals and it will be easier for him to start listening to your aids than to keep cantering. Eventually he will figure out that he better stop before you take him in that tight circle because that's much easier! Then work your way from there when you don't have to circle anymore to stop him
Gosh, never resort to a stronger bit. Even if he responds to that, his mouth will eventually harden to that as well.
I agree that he needs to get his teeth checked. If everything is okay there, go back to respect training. Get him responsive and focused on you by lunging and changing directions often. Practice leading and every time you stop and he keeps walking, surprise him a little bit and back him up a few steps. Backing in and out of stalls sounds simple enough but it has worked wonders on my guy. I didn't believe it until recently I was bored and decided to give it a shot. Amazing. Yielding is also very important.
Yes I suggest maybe a float. I don't know your level of riding but, If your tense your horse will be to. He'll play off of you. Try transitions. Is the balanced enough to slow him self down from the canter properly..... Get back to basics get on the ground lunge him canter him without a rider and the do transitions from the ground. Also maybe a chiropractor...
Maybe have a dentist or vet check his teeth. You could also try adding a light tie down and do some exercises to help soften him up.