Firstly, get that bit out of his mouth. As Corino said, is he isn't stopping in that, strength isn't helping you one bit.
Get him in a smooth mouthed bit. I prefer a snaffle for my training work, but if you must use a leverage or gag bit, then go for something without huge shanks or endless gag action.
Stopping is a really easy thing to teach a horse when you know how - but really easy to stuff up if you don't.
There is one, super important thing you need to do, and it's the key. Without it, you will NEVER have a good stop.
As soon as, and I mean not a millisecond after, that horse stops - You release all pressure. Straight away. Doesn't matter how much you had to get in his mouth to get the stop, you need to give it all back.
The second key - Start with your ideal cue, and escalate to whatever it takes to get the response.
This is how I teach a youngster to stop in the first few rides.
I get a good walk up on a loose rein. Firstly, I sit up, clench my butt cheeks, and say aaaand - Whoah. The 'aaaand' is like a half halt with my voice - And eventually, they better be stopped on whoah or they will know about it. But not yet - You can't expect it straight away.
If there is no response, I pick up the reins lightly.
No response, I take a good hold on the mouth.
No response, I flex the head around to the side and let them circle until the feet stop.
As SOON as the feet stop, I relax, give the rein back, give them a pat and let them stand and think about it. Then I go again.
Horses are smart - it doesn't take long for them to catch on. Latte figured out in like 3 rides to stop off my seat and voice by the time I said whoah. She now stops almost better than my 7yo gelding who had some 'questionable' training.
I don't pick up on their mouth too much until they are clear that the bit means stop. That would be pointless and confusing.
But on a horse who knows better? You bet i'll get in their mouth if they don't respond.
So, for your horse - I personally would put him in a snaffle, get him in a yard or a fairly flat paddock, and do similar. Get him walking at a good pace on a loose rein. When I was ready to stop, I would sit up, clench my butt, and say aaaand - Whoah. No response? I would pick up on him softly once. No response? I would pick up on him HARD. If he doesn't stop, I would use a pulley rein - One rein nice and short and lock that hand on his neck, and pull up and back, hard, with the other. Sometimes it's necessary to use your entire strength on the pulley rein, but usually not more than once.
If he still doesn't stop, then I would look into checking he doesn't have any nerve damage in his mouth.
Remember - As SOON as the feet stop, give him the rein. Doesn't matter if his head is sky high - He needs to learn without a doubt that if he stops, the pressure goes away.
If you do it properly, he WILL get lighter. Do it at a walk until he stops without rein consistently. Then move up to trot and repeat. Canter - lather rinse repeat.