Try usingg heavier poles. Thats what  everyone @ my  yard does! 

Trust me it works! 

Once  he hits it, he'll realise he won't want  2 do that again &  he'll lift his hind legs higher and clear the jumps. 

Hope this helps (:

Hey. Just thought I'd give you some tips for improving your horses "backend". Gridwork and bounces are definately good for the jumping side as they make the horse much snappier. Although these are good exercises they tend to improve the front end more than the back. I'd say the best way to get your horse to improve behind will be flatwork and transitions to engage his hindquarters. It wont be the jump he doesnt have it will be the canter as a horse can only jump as well as his canter will let him. He's only young so working on building up his canter will improve his jump. Turn on the quarters and turn on the forehand help improve this too, just little things to build up muscle and impulsion. Make sure he's listening and feeling like a "bouncy ball" before you start jumping and get him loose through his back and neck as this will help him to tuck up and out behind. Hope this makes sense and helps! =] Good luck!

 And lots of contained impulsion :)