What exactly is wrong with his back, I would get that sorted out first, I suspect that is why he is throwing up his head when you back him. If he is in pain I doubt if he'll ever behave, his bad behaviour is his way of telling something is wrong.
I use a stick/wand as an extension of my hand. A dressage whip is ideal because their length is good and they are flexible. First introduce your horse to it by literally using it as an extension of your hand and stroke the horse with it, start at the kneck and slowly work back, if he's nervous reassure him use nice firm strokes, don't tickle him.
Then I fit a chain to the headcollar, through nearside ring, over the top of the noseband, then underneath and out through the offside ring, up to the next ring where the poll strap attaches, through that and back down on itself and clip it back onto the chain, you want enough chain on the nearside to hold but too much in case it would catch them in the face. You want your head collar fitted so that you have 2 fingers width between the protruding cheek bone and the noseband.
Stand at the side at head level, have the chain in your right hand, supporting its weight and have the stick in your left, hold it up in front of his nose so that it is quite visible, and when you are ready to walk on, ask with your voice and gently twist the hand supporting the chain in a forward and upward motion and sweep the stick out in front of the horse (opening the gate) and use it as a sort of guide, point it in the direction you want the horse to walk in. When you want to halt, once again use your voice with either 'whoa' or 'halt', whatever you are happy with and gently twist the chain in a backward and upward motion, at the same time make an exagerated movement with the whip, make a slow sweep towards the horse and 'close the gate' bringing it to rest in front of his chest. By the way the gentle twists of the chain should be very subtle, imagine your lower knuckle is at 6 on a clock the forward twist goes to 9 and the backward twist to 3, after both return to 6, use it as a suggestion, not a push or pull.
If he gets in front of you, or pushes/barges give a good tug on the chain and give a good stern 'NO', circle him and try again. Try and remain calm, always be consistent, always use the same commands. As soon as you see him do one tiny thing right praise him, not with titbits, use plenty of voice and stroke him. Always support the weight of the chain, if you do have to give a sharp tug, make it just that, don't hang onto the chain and always make sure it has released after it has been tight.
Its always best to start this work with a wall on the offside, so that when you 'close the gate' you have the wall to help you. One other thing is if he is walking too fast, try speeding up yourself, but take long exaggerated strides, pretend you are walking on the moon and he may slow down and imitate the stride you are setting, though this might not work at first if you work him regularly like this as he begins to listen and respond he will.
I'm sorry this is so long, but it is fiddly to explain, I hope this helps you, there are loads of different inhand lessons, I find working a horse from the ground like this before I ride them a big help, it gives you both a chance to get to know each other, builds up respect and trust and gets them to respond to you voice.
Good luck
Lesley