Does he canter on the lunge line? If not, I'd start there. And while usually I won't let a horse make the transition from a bad trot, in this case, MAKE HIM CANTER. I don't care if you trot him around faster and faster for an hour. The issue here isn't rushing into the canter, the issue is cantering. Once he knows that he has to canter when you ask, then you can begin to work on more collection. And when you do get him to canter, only let him go a few strides and then bring him back down. DO NOT let him break from the canter, MAKE SURE you ask for the trot.