Do you know how to do a 1-rein stop?
If I'm understanding you right, your horse takes off at a faster pace than you would like him to at a lope, right?
In addition to the collection stuff the people above posted about, why not try this:
Start with a walk. Have a set pace in your mind you want him to go (at a walk) and when he exceeds that pace, 1-rein stop. Sooner or later he'll get tired of having to go in circles and stop, that he'll just not anticipate a cue to go faster, making his gaits slower. Be sure not to 1-rein stop on the same side every time. Switch it up so he doesn't anticipate that either. Once you have him going "your pace" at a walk, then move to a trot, then to a canter, etc.
That's what I did with my pony and he has that 'western pleasure' lope now! :)