The first thing to do, at least as I see it, is to restore "meaning" to the rein. That means that when you put reins on, you don't release until you get a response, and when you get that, you give a real release. I would venture to guess that this horse may have spent a lot of its' life being ridden with the "brakes on". I mean, reins always kind of tight, and never ridden with really loose reins. Also, when the reins were applied, the horse resisted the pressure, so the rider kept it up and they ended in a kind of stalemate where the bit presskure becomes background noise onto which the horse just leans.j
You have to really watch yourself. When you put a rein ON, then you are doing it for a reason, right? Well, get that reason and don't settle for less. If you want him to bend to the right, then put a pressure on the right rein and don't give the rein back to him until he bends. THEN give him a lot of release as a reward. Make the pressure and the release really clear. No gray. ON and OFF. Don't leave the rein in the ON postition, but don't turn it OFF until you get an honest response from your horse.
This is how you build meaning back into the rein. When you are standing, resting, don't have any ON in the rein. Let them be totally OFF. Don't pick up the rein when you ask him to walk off. Reins mean give your head , slow , turn or back up. They NEVER mean go forward. That's your leg/seat.
Really watch yourself and your horse. Make sure that when you put a rein on it's for a reason and don't give up until you get an honest try from your horse.
Don't expect perfection, but he must at least give a real try, not a blaahhhhhh barge down on the rein.
You will be amazed at how the rein can regain meaning.
That's for starters.