Teaching a dog to sit is not hard. Do not push down on his rump.
Get a treat. Put the dog's leash on and stand in front of him. Let him sniff the treat in your hand, but don't let him have it. Raise the treat up and back above his head. In order to follow your hand, he will raise his head and sit down. Just before he sits, say "Sit" and when he does, give the treat.
Agree you should get this dog into obedience school and get him neutered.