Hide the clicker!
Put him on a lead and if you usually use a prong collar put that on him too. Have the clicker in your pocket, having put it there first thing in the morning or the night before. Go to where you usually do your training session and start with something simple like a sit. Have the dog on your left and your right hand in your right pocket holding the clicker, have a few treats in there as well that the dog hasn't seen. If the dog does not obey you must correct him. Give the command more forcefully and when he obeys drop the clicker, grab a treat and give it to him with loads of physical praise and silly voice, put your hand in your pocket and hold the clicker again. Then walk forwards asking the dog to heel. When he is walking properly give him a click without speaking or looking at him. Stop and ask him to sit again and reward as before. Never click for instructions like sit, down, stay until the dog is fully retrained. When he is obeying the sit without the click go on to the down, the stay etc. When commands are obeyed stop using it for normal walking and instead use a quiet, "Good lad." If you wish to continue using the clicker after his retraining use it rarely and only when the dog is unaware that you have it - it is after all just a substitute for verbal and physical praise.