Never say Thank You!

The politicians I have known, and heard of, walk out of a room when they cannot credibly counter an assertion. It's hard to not get provoked with this conduct.

With hormones and all I hope he grows out of this passive-aggressive method of handling reality. . .but. . .to 'return the favor' you could say out loud, "Oh, look everyone! X is leaving the room because he cannot credibly refute what I just said."

Then take cover!

He may then be forced to come up with some other method that is hopefully more adaptive to reality (which you, better than he, are in a position to represent).

Ok this might partially work. But what I meant was that even at the end of a visit, he will just walk out without saying anything much but, bye.
I once spoke to him about the fact he never thanks us for anything. Just enquired what the reason was for this. He said he didn't know but it was difficult for him.

I've read up a bit on this passive aggressive behaviour you wrote about. It does seem to apply, but what I find difficult to understand is if he is angry with me for anything, or if his problem relates to something else but is projected on me/us?

Again, the contradicting thing is that I am sure he appreciates and likes us for who we are. So why the anger?

I'd suggest he just possibly shy and finds its difficult to express certain emotions. It could be caused from a whole range of things from bieng afraid that his thanks might be rejected to him saying thanks the wrong way. Maybe he grew up in an enviourment where emotion wasn't shared. Sometimes it could be his way of communicating making an action rather then expressing the words. the eyes are the key to our souls maybe thats what you could look for

Its just sometimes other people are lacking of confidence saying those words. Somehow, actions speak louder than words.

Thank you!

Thank you
To be honest it not seems to be a big problem

Thank you.

Big word with deep meaning

and it bring happiness.

It would be easier to say if I knew him my self but going off of what you say, I think he's struggling with things. Parents expecting too much, peer pressure, bullying, any one of these things hushes people and makes it hard for them to speak out. Ask him about how his parents act, if they ever expect too much or react in extreme ways, then about his friends and what they do, then about any enemies he's made. If he doesn't crack a smile, weather what he's talking about is the problem or not, something is wrong. I know from experience that parents expecting obedience can make a child quiet, or it could be all genetics if none of the three seems to be the problem, in which case, don't worry about it and in time he will try to thank you.
Pay attention to his eye movements and posture after and during each question, read up on body language if you have to. But don't let him know what your trying to do or he will try to make him self look calm, most people do for fear of being looked at differently for having problems. But again it could just be genetics and how he is wired. I hope things work out.

Have a good day!

I say to look for his bodies movements because I'm willing to bet a dime that if any of it is true, he will lie or avoid the question. So you have to find the answer from his subconscious movements.