My horse was always hollow in the back and drags is feet. I got dressage lessons and worked on lateral exercises to supple him up. It took a few months but it worked. Ozzie was now on his quarters and on the bit, that is what you need to do to get your horse off his forehand, you need to engage his quarters.
I did this with alot of 10m circles, 10m tear drops (do a half 10m circle then head back onto the track of the arena so you're going in the opposite direction), rein backs, walking pirouettes, and school figures like, in the arena place 2 cones on either side of the centre line, so you ride down the centre line then turn left or right and go back to the start and go the other direction, serpintines. these exercises will get your horse using his back and his quarters so he's lighter on the front, he will become softer, in frame and so he will be off his forehand