Sounds like you need to build cruise control. First off, I wouldn't keep pushing him because it gets tiring always having to do that and then when you stop so does he. What you need to do is ask him to trot and then keep your legs off of him. Just think trot. Whenever he walks immediately go through the levels of asking (ask with your body, cluck, then spank) to get him back up to the trot. Eventually he will realize that you want him to stay trotting.
It is best to work on this in an arena or large area. Let him choose where to go, but he MUST go the speed you want. And if he goes too fast an easy way to break that is to one rein stop him and then go off in the gait you wanted to go. He'll learn that he can't go faster than you want because then he has to do more that way.
Does that make sense? If you have any questions feel free to ask