What i do that should help is thinking about keeping the weight in my elbows, not my hands. Hope this helps!
First of all you need to make him see a dentist to be sure that he has no problems.
Then, you can try to ride him without any bite and in lunge/halter. You need to find the control on the ground first before riding him again. When he gives his head in, release the pression right away and reward him so that he will understand what you ask him.
Having a tough mouth means that your horse is contracted so you need to relax him with jaw yielding and poll yielding.
He is probably looking for a balance point so you need to work on his balance and put his weight back on his hindquarters.
Work with less hands that you can and educate him to the legs.
To make him relax you can try a copper bite or one with a toy on the tongue.
I hope i could help you, good luck :)