Pat him on the but and push his hind parts down - As you say sit - When he does this on his own - Give him one of his fav treats- And repeat until you no longer need treats for him to understand - Also - point at the ground while saying Sit- It helps-
It takes time and training. By using repetitive actions and words dogs can be trained to do a lot. As far as sitting is concerned, actions like pointing to the ground and pushing its back down would teach it to sit when u point down. Likewise, you could use commands like 'SIT' to train him too. When done repeatedly, it learns to understand what you mean and obey you.
Don't push his butt down, that's a strength battle you don't have to fight. What you do is get him to face you while on leash, pull up gently with your hand holding the lease, his butt will rather naturally go down as his head goes up. Connect the word "sit" with the motion of your hand, and then give him a tiny treat when he gets it! Really one of the easiest commands to teach as the dog has no objection as long as you are standing there. If you connect a hand motion with each command, you'll be giving silent commands in no time. If you want to teach him to stay, that's the next logical command. (con't)