First I would make sure there is not a pain or mechanical problem. Has his teeth been done recently,has his wolf teeth been pulled, is he sore at all on his feet (doesn't sound like it but have to ask), saddle fit problems, sore back, etc. If all that checks out than I would have to say it is just him being naughty. We have a mare that is very light on her front end, when she doesn't want to due something up she goes. I had a trainer friend work with her for 30 days and put a little more of a handle on her. The first thing you have to due is get the horse moving and moving forward. If they are moving forward they can't go up. Get that hind end moving due circles. I know with our mare in the beginning was she got away with a lot because my husband is an inexpierenced rider and she had scared him by going up. She new what bothered him and pushed the issue. Now He is more confident and can correct her before she ever gets that far. Yours needs to be corrected before the crow hopping starts. Does he act up when he is tied? If so the other thing I would due is saddle him up and leave him tied in either the stall, arena, or a round pen for a couple of hours. It's good for them to stand tied.

He needs to learn that stopping and standing is a reward and if he can't stand quietly then it is time to work.

Even though he is not a big horse it is a VERY Dangerous problem.

Hope this helps.