If he rears, you being a green rider, I would wait for him to come back down and jump off and open a can of whoop a$$. That is NEVER okay. That's a good way to get yourself killed. When I say kick his ass, I mean get the end of your reins or a dressage whip, or crop, something, and chase him sideways with it. Backwards. Lunge him. Make him move until he realizes that holy crap, Mom means business. You would rather that then a horse who escalated to being flipped over backwards. Whether it's your error or not, he should NEVER even think about rearing.
Sure I may sound harsh, but the horse needs to respect you on ground and under saddle. Have you looked up Clinton Anderson? That would be a pretty good idea of what I am saying, and he can explain it a little better than I can.
Just remember, not everything is gentle but not everything is harsh either. Always ask with steady, gently pressure, and follow through with a harsher approach. He'll start reacting to the soft pressure pretty quick.
I really suggest finding a trainer to show you how to do these things...It will make all the difference.
For now, here's some videos from CA. The groundwork is pretty much all the same here, and you can see various disrespect issues that it can solve. Does your horse have bucking problems? - YouTube
This one, you can see how aggressive this guy was getting for a minute there. Do NOT be afraid to do that if you feel you are in danger. Make them think if you get hurt, their lives may end too. On FSN Sunday, August 14th, 2011: Clinton fixes an aggressive and dangerous horse. - YouTube
I remember once my mare Selena struck out at me with her front leg. Never have I seen the "Oh ****" look come into her eyes when she realized I was not having any of that. We ran backwards, and I don't mean a sissy backup, I mean she was flying mach 20 away from me before I even took a step because she just knew she did wrong. I'll tell you though, she hasn't even thought about it since.