Set a grid - a little x, about 30 feet, another x, 30 feet, an in-and-out (jump, one stride - about 6-8 feet, another jump). The in and out distance will be fairly short because you're going to trot it. Start by working the grid at a walk, stopping between fences. Then trot, stopping between fences. Then work trot, just hesitating between fences. make sure he stops on the straight line after the last fence - he's not allowed to run ahead of you there either.
Also work without jumps. Sounds like he gets stronger and doesn't pay attention to your body position and staying with you, so do a lot of trotting without jumping, working on variations in speed, lots of stops, etc, then carry that through to your jumping. Basically, start asking more of him than just jumping the jumps - change speeds, direction, stops, trot a jump, then walk one, etc.
These are all things I did with my riding horses, that also have seemed to work well with the minis (of course, the ground work exercise was flat work with the riding horse, but you get the idea).