How can I teach my horse flying lead changes?

I have a Morgan horse named Timmy, and he's 17. I want to start doing "beginners english/western riding" with him this year, but the pattern for it requires lead changes. I've practiced the pattern doing simple lead changes, weaving through cones. However, my horse requires a little more time to perform his simple lead changes, so I can never weave through the cones right. I usually mess it all up. My 4-H leader and I have worked with him on them, but everytime we finally get him to change leads, he sometimes speeds up way too much and he always leaves his back legs unchanged. I'd like to get him to keep a constant speed, and switch his back legs. Any help here?