Hold the leash over and slightly behind the dogs' head keeping just enough tension on the leash to keep the slack out, but not tight enough for the collar to be pulling on the dogs neck. With the other hand, extend your arm with your palm open and up, as if you where to wave at someone, holding the treat between your fingers. Combined with the pressure of the leash pulling back and him watching your hand with the treat in it going over the top of his head will cause him to step back into the sit so he doesn't lose site of the reward. When seated give him the reward. If the dog starts taking a couple steps backwards with his front legs before he sits, that's okay.
Some dogs walk forward with their back legs into the sit, and some walk back with their front legs.
You can teach them to do it the one of these two ways. Do this a few times until the dog starts sitting without having to pull on the leash. Keep using the hand signal. You are teaching him both the verbal and hand signal at once even though he doesn't realize it yet, and only interprets both signals as one. We will break them down to separate commands shortly.