It’s possible that he shuts down because the strength you need to correct with a flat collar…is extreme. I don’t actually think I can “correct” my dog on a flat collar without some type of hanging or cutting off of the air or blood flow. His pain tolerance is high, and a flat collar correction is just annoying. A prong, all it takes is a slight pop and he gets the point, he gets rewarded for doing what is asked, he’s happy again, and we move on.
If he’s pulling, and you don’t know how to use a prong or want to…you should start the “change direction game.” At that age…you’re going to have a hard time shaping a behavior, or getting focus without someone teaching you how to do it. But what you can do is that the moment that leash goes tight, turn around and walk the other way. Then again, then again. If you end up taking 3 steps and the dog just starts pulling forward again, turn around and walk the other way. If it takes 20 or more turns within 3 squares of a sidewalk, do it. You’ll look dumb, but eventually your dog will get the fact that pulling doesn’t get him where he wants to go, but he’ll eventually get to where he wants to go if he walks at YOUR pace.