What happens if he says to someone, "You don't implicitly believe everything I say, but are free to make up your own mind of the truth of what I say?"

@Frostfyre: I guess they would still believe everything he says, but would be absolutely convinced they made up their own mind about it.

@Joe Bloggs Reminds me of Killgrave in a certain sense.

How much do people believe his implications, as opposed to his literals statements. Interestingly, you may find a lot of source material looking at sage martial arts masters of long lineages. They often have to be very careful with what they say, for fear of being taken at face value.

I think you misspelled Joe. The right spelling is Kim Jong-un.

"I don't get it. Your life is... perfect! How did you get to have such a perfect life?" / "You'd believe me if I told you."

Also, what about paradoxes and other contradictions. What do people do if Joe utters them?

Is Joe's skin purple, and does he have an odd tendency to draw out the word "Jessica" when he says it?

Does the effect apply with other forms of communication, especially writing? Does it apply when filtered through a voice modulator? Is it specific to the person or to the person's communications?

This was the plot of "The invention of Lying".

Learn hand signs then or use a voice synthesizer...

You're going to delete this question and stop meddling in my affairs.

Do people who hear him continue to believe him after he has left them?

If he starts a sentence with "You won't believe this..." he sets up a paradox.

I think the example chosen is poor because 'good morning' is a wish for someone. But assuming you mean something like 'It's a great day!' then the simple answer would be a) to not lie and b) make it clear that opinions are opinions (i.e. I think it's a great day)

See the Church and State storyline in Dave Sim's Cerebus the Aardvark comic for an examination of this premise, among other things. "

I can't believe nobody asked the obvious question: What if Joe says "I am lying"? Or "Every statement I make is a lie"? "This statement is false"? Do other people's brains explode in a shower of sparks and cognitive dissonance?

This makes me immediately think Joe is an SCP. I think I've read more than one SCP with a similar effect. Not that this necessarily helps much to answer the question but browsing the SCP wiki for SCPs with similar effects might give you some inspiration based on how their effects work and how they're tested. Description: SCP-2344 is a male caucasian approximately ██ years old. Any statement SCP-2344 makes is immediately and unconditionally taken to be true by anyone who can hear and understand it.

@CortAmmon Indeed, Dan Djurdjevic has written on this effect in martial arts. Joe could end up having an effect similar to mentalism on those around him, possibly convincing even attackers that they will drop dead if they hurt him and have it happen, even if it's just them believing it too hard. Could have further implications, such as unintentionally starting cults and religions when he says things.

How does Joe avoid temptation? "You're the most beautiful woman I've ever met and we're going to go up to my room and make mad passionate love". (oops!).