Learning to Sit Nicely
If he does, he will do it every time you ask, the first time you ask. Teaching your puppy a reliable "sit" cue is very important. If your puppy is sitting, he is not jumping on people. The secret is to teach your puppy the action before you teach him the word. To do that, you'll need to have some small bite size treats handy.
First, hold the treat in your hand between your thumb and index finger. Then keep your palm open, facing the ceiling. Hold the treat slightly in front of your puppy's nose and slowly lift the treat above your puppy's head. This will cause him to look up. Then as your puppy looks up, he will sit. Reward your puppy as he sits.
A pet trainer can help ensure this is done properly so that your dog truly learns the cue.