Using his hind?

Rupert doesn't use his hind end when he canters. Is there any way I can remind him without a whip?

What are you doing with your seat? Upper body? Legs?

I give him plently of leg, but I'm basically clueless..

Lots of transitions help engage the hind end. Trot poles also work well. Once he's going over trot poles consistently well, raised trot poles. My OTTB didn't used to use his hind end either. After tons of the above mentioned work, combined with trail rides in the woods to get him thinking about where to put his feet, he is now trotting small cross rails instead of jumping them since he had now figured out that he has 4 legs and not 2.

From a rider standpoint, make sure you are not riding in such a way as to hinder his movement. Make sure you're centered over his center, not leaning over and loading up his shoulders. To test whether you're centered, trot on an insanely loose rein so you have nothing to hang onto, and post so that you are down for one beat but up for two. If you're not centered, you'll either tip forward or fall back. Initially do the poles in a light seat so as to stay off your horse's back while he's figuring out this hind end thing. Also make sure to use heavy poles. You don't want them rolling when he inevitably trips on them or try to plow through them.

We're working on the transitions with him, and I try my best to exagerate the loose rein. =] I do tend to lean on my hands, so that might be an issue for him.
We have the arena set up for a coming dressage test, so we can't go in the middle of the ring, where we'd normally put poles, but once it's all cleared up we'll probably work on them more! He has breathing problems when it's hot out, so I can't ride him as much lately. :P

Can you get video footage at all?

I just wanted to add - I am sure Spyder and CJ8SKy and Koomy will have excellant advice as well.

Rhythm
Relaxation
Contact
Schwung
Straightness
Collection

When you are doing the transitions, do many of them and do them correctly. Sit and ride the horse through them.
First prepare for the transition, keep the horse round, forward in rhythm and stretching to the contact, then perform the transition keeping the horse in rhythm, stretching to the contact and going forward. Then in the new gait, rebalance and retain this.
Then repeat a few thousand times.

That would be helpful.

As soon as I'm back on him. Pour boy can't be exercised much during the summer, it strains him a lot :\