Well you put this question in the NH section so I would just talk to him.
Welcome to the forum, cpc417!
Firstly, I'd say only ride in an arena or roundpen for now. No brakes are no fun out in the open. Also, only walk until you have solid brakes there, and then move up to the trot. Does he stop willingly on the ground? If not, start there, he needs to know that anyway as a matter of good manners, and it will help him under saddle.
On foot, lead him forward on a loose lead, and stop your feet and say "whoa." I use long vowels and a low tone of voice. Slowing gait and stopping get low slow words, and speeding up get quicker, slightly higher pitched words. If he chooses to walk past you, take the tail of your lead and move him faster, disengaging his hindquarters and making life generally harder for not stopping. When you have his attention, ask him to stop again. Repeat until he stops. Practice until he stops with you the first time, every time, with no pressure on the lead. Try stopping from a trot in hand, following the same progression. He should be able to do the same at a walk as at a trot in hand.
Under saddle, I'd do the same kind of thing. Let him walk forward on a loose rein, and allow him to walk with your seat (no pumping him faster, no "blocking," just ride the walk). When you're ready, stop the motion of your seat and say whoa, gently squeezing the reins, in that order. The word should get his attention if you've done your homework on the ground, but he'll probably walk through you anyway. Pick up on one rein halfway down its length and slowly and smoothly bring your hand to your hip. The care with which this is done is imperative. Pulling one rein takes away his ability to lean on you and the bit, but it does of balance him. NEVER do the one rein stop at a dead run (i.e. A runaway situation). When he stops his feet, immediately release the rein and praise him. Be sure that the only time you touch the bit is when you want a stop or a downward transition. I see a lot of horses with no brakes because the rider is always in their mouth: the pressure starts becoming meaningless. Ideally, if you've progressed from seat to voice to rein in your cue, he should start to stop without any more than the seat cue.
Backing up can help, too, on the ground and under saddle. The better he backs up, the better he'll do most everything else.
I really advise at least taking riding lessons with this horse, if not pro training, if you aren't already. A response from someone online can only go so far. Someone knowledgeable should be there in person if you don't have experience dealing with this kind of problem; pro help can be invaluable, especially with the timing and feel needed to build a reliable stop on a horse.