Hello Madelyn,

First of all make sure it doesn't come from a back or withers pain.
Then you have to be careful whan get off that you don't touch him with your leg on the croup.

If it isn't painful and that you don't touch him when you get off, I would suggest to try getting off several times during session and when he tries to buck, severly say no to him. When he behaves good, gim caress him or give him a treat and give him a break.

Good luck!