I have found that the best way to start teaching behaviours is with a visual cue. After all, dogs are highly engaged by movement. If you believe your dog already knows the sit cue, but does it “only when he feels like it”, then I suggest you start the training process again. If you are relying solely on the word, then a visual cue will be helpful.
The visual cue that I use for the sit behaviour is a suggestion. You can use another cue if you prefer. There are no firm rules on what cues to use, just as long as you are consistent!
Pick a location where your dog will be able to focus.
This video is a demonstration of teaching a puppy to sit with a visual cue.
You will notice that I did not say “sit” when Leila was following my visual cue. As this was our first session, I did not want to risk giving the verbal cue and then not have her do the behaviour. After a few more successful sessions I would start to add the word along with the visual cue.