Get your BF off this horse/pony.
Dead mouth and sides.. what that really means is the horse has been taught to resist ALL aids. The horse feels what you do.. but braces against or ignores your aids.
Having had a horse or two like this, I typically put the horse in thick snaffle and a flash or cropped nose band. ON THE GROUND I would ask, with a leading rein, for the horse to give to each side. The noseband supports the bit on the bars of the mouth and helps to prevent yawing.. so the horse needs to pay attention. Just a little at first. This allows the horse to accept rein pressure and give to it exclusive of all other inputs (like you sitting on the horse's back or forward movement).
For dead sides (after getting a yield with the bit) I used a dressage whip and pressure on the ground and a barrier in front of the horse (chest high is nice). Bring the horse up to the barrier and ask, with pressure, for the horse to move his hind quarters AWAY from pressure behind the girth (not WAY back as WAY back can make them want to kick). Back up your request with the whip (not talking about beating the horse.. just tapping very precisely and insistently).
Eventually, take the horse away from the barrier and use the rein to keep the horse from moving forward and allow him to turn his head as you put pressure on his side and ask him to move his hind quarters over. NOW, if you have done this right, he will turn his head softly and move his hind quarters over... and the instant he starts to move his hind quarters over (is committed to taking a step) you release all the pressure.
This process can take a few days.. and in the meanwhile, stay off the horse's back.
When you get all this going for you, try it when on his back. Use the barrier (I used a hitching rail) and start with just the rein request and then combine the rein request with the side pressure. Eventually, get away from the barrier and try it.. and see what you get. IMO this is the ONLY riding you do until you get at least 90-180 degrees of a 360 degree turn on the forehand w/o a barrier.
When you have this, start him over under saddle with serpentines, circles, spirals and so forth. Use TWO dressage whips and when you ask with your leg, back it up with the whip if he resists. Start slow.. walking is fine. You will need to retrain his brain AND his muscles to be flexible and non resistant. It takes time.
One horse sticks out in my mind that I did this with.. and in the end he became one of the lightist and most responsive animals you could ride. He would swap leads every stride and even learned to do a little levade. His half pass and his shoulder in/out was very responsive.
He was build like a plug.. and his conformation stopped him from ever achieving much.. and he had an osselet in his left front. He had no value other than meat.. but I had him for almost 20 years.. and he never was dead mouthed or dead sided after retraining. He was a horse I could get all this from with a simple rope around his neck and no other tack.. and the only horse I never sold. He died on the farm.