Don't worry, there's a fine balance between shutting down a horse and impulsion. You will get that back end working when he finds that impulsion from moving out of the back-up. This can be corrected with the use of a Dressage crop. Usually when I'm working on slowing my horses, I'll do a lot of backing and transitional work. Sometimes they get hesitant about going forward after that, because they think they'll just be made to back up again. Use a Dressage crop to back up your leg cues, and you'll start to feel the impulsion better. I'll come around, stop, back back back, and then immediately ask for the forward lope with the aid of my crop. When they listen to your leg immediately, you will feel them round up from the backing and step elegantly up into the lope. Then after a while you'll have to deal with them getting a little worked up, and that's when you can lose the crop.