I didnt read this whole thing, only the first few threads. But what i would do.
Put both reins in one hand, reach back and pop him in the butt with your hand. Grab the cantle once you do so if you are worried about a reaction. This will keep your but in place incase of something happening. If he takes off dont pull back and stop him, let him move forward. After all that is what you want right? He is only doing what he is told, so if he breaks into a canter let him canter a few strides, break down and ask for the extension again. Sounds like to me he is rather feel-y. So riding with a crop or something along those lines might not be needed. He just needs a little wake up call. Make sure when you pop him in the butt you make a sound such as a cluck or say "trot". Do this a few times so he associates the sound with the pop, and he will move. This eliminates the hassle of carrying a crop or a whip. Along with him "freaking out" as much. I would also suggest spurs, they will help reinforce forward motion. Good luck.