I would do a few things...
I would try to keep leading sessions to a minimum five-ten minutes but maybe do a few several times throughout the day if possible. However, if he starts rearing during the short session do not stop. How is he with giving to pressure? And is he desensitized to a short crop? This leads into my next suggestion
If he starts rearing, turn him around, using the crop to help move him into the circle, I'm not saying lunging, but to just turn him and walk the other direction and if he rears again do it again. Let him know that if he does this while you are leading him it is going to mean more walking/turning and the crop is not used for discipline but just to give you your safe space from him.
Hopefully by shortening your sessions, since the young ones have little attention span, he will learn that he can rear on his own free time but that it will not be tolerated during handling.
I'm sure others will give good tips, but please let us know how it goes for you!