i would definately go back to using the collar and lead. if the one you have slips over his head, put some smaller holes in it or buy a smaller one for him while you train! the harness only gives him power and at the moment you need to be in control to make him happier when out on walks.

you could also just use a simple rope lead and put it as high up his neck as possible, so that you are in control of his head, neck and body.
when you do this he will be on tip toes and you will be able to move him where you want.

he may struggle and also make sounds as though he is choking and as though he is hurt, but this is just a mechanism that dogs do, so you stop what you are doing to them. this doesnt mean you are hurting him!

i have put a new album on my profile of some pics with dogs and the position of the lead/collar for you to have a look at, so you know what i mean.

once you have him tiptoe walking when on the lead, he will not totally cooperate at first, but once he has gone a few yards he will start to loosen up, you then rewad and praise him.

to stop him constantly looking back, you can try talking to him, (doesnt matter what about) or take out his favourite toy for him to focus on, this will destract him. when on walks he needs to be focused on you and what you want him to do (I.E - him walking like a normal dog!)