Typically a green horse that has trouble cantering is having confidence issues. I would just keep doing lots and lots of trotting work and one day, when he breaks out into a lope, be ready for it and lope for a while. If he slows, whack him on the hind end with the end of your reins, once he resumes, pat him, relax and lope s'more. I just wouldnt worry much about it. One day, he will figure it out. I love the hill idea, but see if he breaks into the lope on his own, without you distracting him by pushing him.