My horse bolts after jumps.?

I ride at a lesson stable one a week. I ride a 25 year old frisky gelding and after every cross pole he will gallop and I don't have the space to let him do that. The jump is not very high maybe 5 inches tall. He trows me of balance and I will lose my stirrup. Any suggestions? Please do not be rude about his age. He is my favorite horse in the world.