I think the most important thing to start is relaxing him. If he's a little too nervous maybe it will take some time of working without pulling too hard his mouth and making circles to let him stretch out
Try backing up in a straight line. You can also try to get them to pivet.
If you lunge them frequently and build up muscle then they will use their backend a lot more
Maybe try lots of shoulders in and counter shoulder in on a circle? This will really get them moving behind and listening to you leg