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!
*are also very good but since it focuses on the inner hind, make sure you do both sides equally.
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
Lots and lots of lateral work, or try long lining where as he/she gets better, they'll collect more. This means yay hind end engaged! Good luck!