Circles, Trot poles, loose reins, spirals, leg yeilds, basically anything to get his hind end moving and stretching! My horse is exactly the same and I find these help him a lot! Topline will get better with these as well!
Distanced ground poles are great to help him reach. Also make sure you leave his head free most of the time where he does what he wants. A restrictive head set worsens the trait.
Small circles (5m) at trot or canter