Transitions. Of all kinds. Walk to canter. Canter to halt. Trot to walk. Etc. You get the picture. You will know that he is shifting his weight to the back when the transitions become smother and lighter.
It seems to me that a lot of horses become heavy in the front end when they become bored. I would suggest cutting jumping out of your routine for a while, and just ride. Trails, arena work, maybe down the road. Cut all of the regularity out of your rides. Do LOTS of transitions, and be very sure that you aren't pulling your horses head down "into frame".