Your seat alone will not make a huge difference in his body positioning.
If you are serious about getting him on his hocks, you should start schooling your side-passes and haunches-ins.
When I have a horse that's heavy on the front, my goal at the lope is to get the horse off the inside shoulder, bend the core, usually by side-passing at the lope. Then I might continue on at the counter canter, straightening the horse out and pushing up the hind end. Then I'll do another side pass on the diagonal to get back onto the correct lead, and then once the shoulder is up, I'll ask for the hindquarters to come up in the haunches-in at the lope. Drop the reins and bam, you've got a little rocking horse.
Of course, that's the end goal. Everything starts out at the walk. But I can tell you, it takes more than a seat position to get that good canter.