Are you riding English or Western? Long rein or contact? I could help you with the western, someone else might be better equipped to help you with the english
If you are riding western, lope on a long rein, hands down in front of your pommel, relaxed and sitting on the bones on your butt. When you feel him going beyond the speed you're comfortable with, sit back on your pockets and push pressure down into your stirrups, if you feel him slow for you, that's great, now relax (both of you) and run through that again. If he does not respond to your sitting slightly back and into the saddle on your pockets as well as legs pushing into the stirrups, pick him up (reins) and make him round-up but be sure to keep some heel on him to maintain forward momentum. You will notice as he rounds he will slow down. Let him do this for a few strides and then let him go and relax. If he picks up speed again, repeat the process : weight in stirrups, sitting on pockets of seat and then picking him up if he doesn't respond.
All these maneuvers are done in a slow fashion, when you pick up your reins, pick them up in a timely manner as to correct him but don't jerk or yank. Imagine your are slowly doing a bicep curl with your reins.
Keep this consistent and he will soon figure out what you want. Always remember to make the reward for the horse ten times bigger than what he gives. If he does this for you the first time, I would let him stop, pull that saddle of and let him go relax.
I took my horse to his new barn yesterday, and he was super fresh and looking around every where. I took him in the arena and he was super energetic and I actually had to give him a refresher course in this particular way. I was loping large fast and wanted to come down to small slow......needless to say he was a gentleman
Hope that helps.....