It sounds like both you and he should know how to transition to and from a lope properly then. One exercise that may help is counting strides and then changing gait. For example: walk 5 strides, trot 5 strides, then walk 5 strides. Do this at walk/stop and walk/trot first to get him into the mindset that he WILL be asked to change gait soon, so he's expecting it. Then start to include the canter/lope. So walk 5, trot 5, walk 5, trot 5, canter 5, walk 5, or whatever combination you like. The idea is for him to be thinking and ready to LISTEN immediately after changing gaits rather than rushing off like a fool. I did and still do a lot of it as it is a great exercise for both horse and rider.
Another option is to MAKE him run when he decides to rush off, but you have to know your horse and be comfortable with it. My guy would get himself spooked when we first started cantering and then wound want to run off rather than listen to me. So I let him. He wanted to slow down and stop after about twice around the arena. I made him keep running. And then I made him keep running another lap. Then I asked him to stop. If he listened, he could rest, if not, around we kept going as I make him keep up the speed until he was listening and just waiting for me to ask. As I said, this works well with horses who are essentially lazy, not as well or quickly for some fit horses who just love to go.