My TB rushes fences and cat leaps over jumps, help!

OK, my new loan horse is an ex-showjumper, he's fifteen.one and he can jump over one metre twenty. I'm loaning him at the same yard he's been at for the last ten years so he hasn't had the disruption of moving or anything, and he doesn't have any injuries and he's about fifteen so he's not young or green. I jumped him for the first time a few weeks ago (when I say jumped, I didn't put up a jump right away, I just balanced a pole on the jump wings about 10cm from the ground, on account of all the cups having gone mysteriously missing) and I expected him just to trot over it, but the second he saw it he stuck his head in the air and literally galloped tiwards the fence and leapt it almost a metre higher, despite it being basically a raised pole. I came off as I wasn't expecting it, but the next few times I turned him towards the pole, he did the same thing, except after he's jumped it, he just trots away as calm as you like. And a week ago, I was in the arena again, and someone had put a course up. There was five, metre.ten high jumps up but Buster was weirdly calm, being all sweet and responsive as I trotted him between the jumps. The second I nudged him into canter however, he did the same thing and tried to jump one of the jumps, but I pulled him off just in time. Its starting to make me worry, and I really want to start competing him over the summer, but I don't feel confident enough. Any help on whats making him do this and what I can do to prevent this in future??
Thanks :)

Set up lots of gymnastic grids and work on maintaining an even rhythm with him. I'd highly recommend the book 101 Jumping Exercises for Horse and Rider by Linda L. Allen. So many great exercises in there!

To me this sound like a horse that has been jumped and jumped until he doesn't know anything else. If a horse is expected to jump every time he is in the arena he is not going to understand when you don't want him to jump. He doesn't understand the concept of flat work because it is never something he was asked to do.
I may be completely off base but that is the impression I get from what you have posted.
I would just work with ground poles, try to avoid having jumps in the arena for the first little while.
Set up poles all around the arena and just walk over them, go around doing circles, figure 8s etc and keep his mind busy, every once in a while just walk over a pole. Keep doing this until he ignores the fact that the pole is even there then move up to a trot. He may try to jump them like a madman at first but just keep consistent and practice until he is walking/trotting calmly over them. Work your way up until you can do the poles at a walk trot and canter without him jumping, then set up a very small x, small enough that he could walk over it without jumping. Walk, trot and canter over it until he is calm and quiet then slowly start raising it up.
This process might take days or weeks depending on how well he responds, don't rush it.

I second what LynnF said. Start right back at the very basics. A single pole on the ground. Start in walk, ride over the pole do figures over the pole, halt 2 strides before the pole, walk over it, halt 2 strides after it, and so on. Once you can do that in walk calmly then repeat it all in trot, once trot is easy and calm, canter. If you are happy, relaxed and confident, add another pole at a related distance, and repeat everything again at walk, then trot then canter. Don't lift the poles off of the floor until you can canter a course of poles in a relaxed manner. Then start lifting the poles off of the floor.

Rushing jumps and cat leaping is also associated with pain and fear, so I would suggest you get your saddle / back checked first.

Thanks :) I've been trying them while lunging him and it worked very well, he was fine in walk, leapt them a bit in trot but soon calmed down. In canter he threw himself over them in his usual style, but after a couple of sessions he was absolutely fine. Haven't jumped him since, just schooling him mostly now and he's actually been a bit calmer, which is lovely, so thank you.