There are so many things that can be a part of a horse that is not responsive to the bit. First, be certain that his mouth is not in pain, bit fits, saddle fits, teeth are floated.
Once those physical issues are ruled out, then look to HOW you apply the rein aids. Usually , what happens is that over time the rider (either you or his former riders) are allowing dullness to develop by asking for something and accepting less, over and over and over again.
YOu may be really apply a ton of pressure, but if you accept a very limited response from your horse, and you release the pressure there, you have built a great deal of tolerance into your horse, so that every time you will need to use a ton of pressure just to get a half ton worth of result.
So, you have to start babysitting yourself , first. You watch yourself and EVERY time you ask for something, do not accept a faint shadow of what you asked for as a result. If you ask for the hrose to STOP, then do not release the rein while the horse is still leaning on it. You have to hold the rein , with a kind of pressure that meets the horse's resistance and add ONE OUNCE MORE, until the horse gives, and backs up. Don't accept him leaning on the rein. Wait for your release until he actually comes off the rein, then release.
If you already know how to use the reins, then use them, but watch yourself and make darn sure that if you ask fro something (stop, back up, flex right, slow down , etc) that you do not reward a half response.
Then you start asking with a lighter "ask" and see if the horse will respond there. If so, you are now building in lightness instead of dullness.
It all depends on your level of expectations, and how willing you are to hold onto that and require the horse to meet you there.