I agree with below.. but if you've had him checked over then i'd suggest first of all making it fun for him! If you're practicing with him in the school then first lead him aound and have a few poles here there and everywhere. Make him walk in and out then (not over), and over them, and round them, and stopping half over them and so on.. so they're not all about hard work and jumping!

Then just raise one side, doing the same and trotting with him inhand... (have other things too like bickets etc to distract him and use these as toys too) to keep him occupided. Just raise em little by little and do the same whilst on it and before you know you'll be soaring!

Hope I made sense to ya.. good luck!!