Rearing is usually the horse's way of telling you they feel they cannot go forward.
So the first thing you need to do to "fix" it is to make sure you haven't closed all the doors against him. For some horses this might be a little too much contact with their face as well as too much leg... or it might be simply too much leg.
'Chronic' rearers are horses who have learned to anticipate that they "cannot" go forward. They need to be re-schooled so they understand they CAN go forward. I generally start this work on the ground... before getting back into the saddle - leading, lunging, and ground-driving, when I have established a good foward on the ground I get up in the saddle and do it there as well.
Then there is rearing related to pain... a horse with back pain or saddle fit issues may rear... a horse with teeth in need of floating may rear, or a horse who has been hit too hard in the mouth may rear. The only solution to this kind of rearing is to fix the pain.