First thing I would do is ground work with whoas. He will need to be retrained a bit since show horses are basically a lot of go go go and not much slow. Just you tube KMSHA and UMH shows and you will see. I also think it will help you a lot in seeing how they sit their horses that give them the cue for faster forward motion and how they relax giving the cue to slow.
He just doesn't realize you are someone who isn't a show ring rider and I am sure since he's a Rocky he will be smart enough to start picking up on your cues. In time he will desensitize to you flopping around on him and figure out you aren't ready to travel at top speed. My husband has a barrel racer who was very sensitive, but now that she has learned we aren't racing anymore anyone can ride and and flop all over on her. But once my husband gets on her she is ready to roll.
We also have quite a few show people on here that may be able to help you with the cues they use.
Do you have someone who can lead him with you on him? I will tell you that even though my 5 year old is super well trained every spring when I take my first ride my husband leads us around the round pen a few times until I feel my confidence build. We are up north so my guy stands all winter so every spring it's a bit of starting over. But once we go around a couple of times I can just feel he remembers me and off we go just like winter never happened.
I do have to say I think you are pretty darned gutsy getting on him and riding in such a short time.