My horse was like that in his early years. I'm just going to let you know, there's no simple method. The best thing to do would be to constantly, slowly improve him over time.
Other than that, I found holding his head very collected while loping to be helpful. It will force him back on his hind end if you're careful to keep him moving through the bit despite the heavy collection. Make sure you keep the drive on him though, otherwise he will drop his shoulders and slouch around. Just keep pushing him through the bit and maintain emphasis on his hind end. Release pressure when he begins to relax into an easy going lope. You should try to slow his cadence down without killing all of his extension. Once he is going nicely, hang your reins out and keep him moving with momentum. Always, always be lifting his shoulders. It sounds weird to other people, but a lazy horse will not lope well.
It's also good to do it in circles. Tighten as needed.
If anyone has any other advice, feel free. It's been a while since I had to slow a lope.