Hmmm... Usually I do circles and pull up on the inside rein while pushing with my inside leg to get him to balance out. Try to focus on pushing him onto that back hind leg. I'll then use my outside leg a little behind to push his butt in.
I also like to do a lot of backing up, rollbacks, and pivots. Work on the straight and then choose one of these when he starts to get heavy, and at all gaits. The focus is on him getting that rounded topline and getting over his haunches.
Then I will do a lot of two-tracking and transition work. Once you've gotten good at the sidepass, pick up the inside and push the shoulder out while pushing the butt in. Getting him to bend his body and use that back end will help.
I will also employ a lot of fast trotting and sidepass. Fast trotting will help build muscle mass, especially if you keep him up in a loose frame and focus on collected, floaty strides. Half passing at the trot will help him balance.
Hope this helps. :)