stopping at his back

Question
I am working on really driving my horse forward into the bit, to put him together. he's coming through really nice on his shoulders and poll, but he's still not getting completely under himself. I'm trying to really drive and do a lot of lateral work to get his hindquarters engaged, but he's longer in the back and just hollows out. Any thoughts?

Answer
Dear Stef:
I think you may already know what your horse's main impediment may be - it is confirmationally difficult for horses who are long backed to round longitudinally. Depending on how long backed your horse is - you could be waging a losing battle.
Frequent changes of direction, spiraling downward from a 20 meter circle into a ten meter circle and then back out again trying to ride off of your leg may help.  Travers and shoulder in may also help you to engage him behind.
Best wishes,
S.Evans
PS - apologies for the delay in responding- I had a flood in my basement and lost my computer equipment.