Transitions and Trot poles
A lot of transitions done the right way. Take help from a trainer who can help you with diffrent exercises
EventionTv on YouTube has a very helpful video on engaging the hind end.
try using draw reins and work on circles while in a frame. pole work, ask ur trainer!
Agreed that lots of transitions are helpful to engage your horse's hind end. If the horse isn't used to using his hind end, doing exercises that will help him build stamina and maintain balance will be beneficial. I recently had a lesson on a horse that was not used to doing a lot of lateral work or engaging his hind end, but he was also very quick and strong. Ended up working on collection and transitions and incorporating ground poles. I think incorporating poles can also be a great way to start building up those muscles. I wrote out a couple exercises that we worked on to get him to step up under himself. Maybe these could be helpful for your horse too! http://www.inboundequestrian.com/weekly-riding-exercises-collection-at-the-trot-and-canter/
Tap it’s rear end with a crop
Transitions, leg yielding , half passes, shoulder in, and many differences in tempo ( trot , medium trot and extended trot and the same in canter) also good thing to do are walk pirouettes.
To résolve your problem of engage the hindlegs of your horse, the best exercice you can do it's doing lots of transitions with a well balanced ! Also you could insist on your rhythm
Good luck !
Doing lots of transitions