If you've read any of my posts this will seem like i'm a broken record but these things are two of the most important things your horse can learn.
Does he flex on the ground and yield his hindquarters? If not, go ahead and teach him, it will make life much easier. Stand at his side while hes in a halter and just take the slack out of the leadrope and wait for him to give his head to the side and for his feet to remain still. You want to make sure you are just holding and not pulling so that you do not continue to pull his head further around once he tries. Don't expect him to be able to reach all the way around to his side, he's older and possibly stiff. As he gets better over a few days you can ask for more and more. Mind you it isn't important that he stretches really far around, as long as he is soft about it, repeat on both sides. Once he gets it in a halter you can move to a snaffle bit.
He also needs to know how to yield his hindquarters. Bend his head around abit then either with the end of your lead rope or a training stick tap or twirl it in the air. If he doesn't move then move to tapping, tapping harder, whacking etc until his butt moves over a step and his inside hind crosses in front of the outside. As soon as he does move, quit putting pressure on him.
Afterward go back and desensitize. You don't want your horse afraid of your tools.
It's quite possible he already knows these things so it could just be a matter of a quick refresher for him or you may spend a day or two teaching him.
From there you have a couple tools in your bag to ride with. If you've been riding him in a curb ditch it and go grab a snaffle.
Go ahead and mount, flex each direction to make sure that is good. Then when that is good, slide your leg back and ask him to move his hind quarters, don't kick if he doesn't move, just press with your calf and then use the end of a split rein and lightly tap him on the butt. When he moves, release your leg but keep his head flexed and wait for him to stand. You want him moving his butt around good on each side before you ask for any forward movement.
When he's moving around better, put more pressure on him so he moves his feet more until his front end gets unlocked and he wants to walk a small circle. At that point Let go of his face and let him go forward.
After a few steps reach out and bend him to a stop to make sure that's working when you have forward movement. Steadily let him go farther and farther without bending him down, making sure to bend each direction the same amount. When you bend down to a stop, make sure and slide your leg back to ask that hind end to move around before he stops. This is important, a horse crossing over in the back isn't a horse that's prepared to bolt.
If you feel him about to move up into a trot, bend him down. After a few times being bent down from a trot you'll feel him start to relax before he stops when he's just walking. At that point don't completely stop him, turn loose of his face and let him walk on again.
It's important you don't pull him around, you want to be nice and soft about it, you also want to reach outward a bit, not straight back. At no point are you using both reins at the same time, that would give him something to brace against.
When he's good at a walk, gently move him up into a trot. If he starts trotting faster, bend him down until you feel him relax. As you add speed, horses get more excited so it's easily possible you may need to bend to a stop again and start over at a walk for a little bit. When the walk is good again, ask for the trot. Again, when he starts to speed up, bend him down until he relaxes. Once he's relaxed, ask for the trot again. After a couple of times of this you should be able to instead of bending to a walk, just bending back to a jog. Each time you ask for the trot he'll trot longer and longer without trying to speed up.
When your trot is good, repeat at the lope.
Before you can start doing anything else you need to be able to w/t/l on a loose rein without him getting uptight about it.
