What breed is he and how old is he? If he is only doing it at the lope, I wonder if maybe you have changed the way you are riding him if he was very good at first and has started to regress. Have you gotten a different saddle, changed bits, gotten spurs? just trying to gauge what you are doing to see if I might have any tips to help you out. What setting do you normally ride in? A good exercise you can try is to start trying to push him into the bit. What I mean by this is:
start by bridging your reins (if you ride western, if you ride english then that's already taken care of) and setting your reins at a certain length. Push him up into a trot, and push him forward using your legs. Set your hands in a "wall" (meaning make your hands solid, do not let him jerk your hands forward) and apply steady pressure. You must keep constant pressure with your legs as well, really pushing him forward into the "wall" formed by your hands. What you are trying to do is to get him to give to the pressure in your hands. When he does this, his head will come down and back. You will feel him slow down somewhat and his weight will transfer back underneath your saddle. When he does give to this pressure, release your hands, giving him his head back and letting him relax. In this way, you are rewarding him for giving to your pressure. You can repeat this process, at the trot and then once you feel comfortable with that, the lope. The idea behind this is that, eventually, when you want him to slow down at the lope, you can just take hold with your hands, push with your legs, and have him slow down for you.
He may also already know how to do this. This is just one idea I have. Good luck!