Say if the dog knows the command very well but ignores & refuses to do so, give a sharp pop with a choke chain & command him to "Sit" again & when he does it, do NOT praise, do not say anything. Take a few steps forward & ask the dog to do the command again & when he does it within 2 seconds, praise. If he ignores & refuses to do it again, immediately correct him.
I find that positive enforcement works a lot better than than negative, so it might be worthwhile to first associate good things with the correct action. How can you correct a dog if he has no idea of what he is supposed to do?
I would say 'sit', while doing a hand gesture that I want him to know as my sit command (while making sure that he's focusing on me) and push his bottom down. Then reward with treat.
After a few rounds of this, if he is particularly stubborn, you can use the collar to gently pull his weight down. The best way to do this is to have the leash in the back of his neck, so you tug downwards, using the collar and leash to nudge his back and hind legs into a bending motion.