Most probably, A., he knows that you are the safest spot in the entire world, and this is where he can experiment.
Nothing wrong, absolutely, he goes through the stage of investigating, and he is not trying to annoy you or make you feel bad. In fact, as I said, he knows you are the safest person in the world, so in a strange way, it is a great compliment to you.
At diffeent ages, my son went through the time when he was telling me ALWAYS "yes-no", no matter my question or sentence that I posed him. He just said "Yes-no", or "I will- I will not" , "I want - I don't want".
"Do you want sugar in your tea?"
"Shall we go fishing this weekend?"
"We will - we will not".
And, I could not get any more reasonable response from him for MONTHS. I just had to make my own decisions.
Then, he grew out of it and started speaking normally again.
I guess, he was in the stage of investigating somehing, and did not make a big deal out of it.
Now, for you, what to do?
1. you van join the game, but do not make it upsetting, know when to stop, to show him only that you ALSO enjoy this research, so to say. BEFORE he gets a chance to oppose you, catch the moment and do the same to him: he will definitely say something, any word, and you PLAYFULLY turn it around. If he will not get VERY annnoyed, you may continue adopting him game for longer. See what happens.
2. BE CREATIVE! For instance, you can QUIT TALKING for three days to a WEEK!!! Write him notes, hug him when approving, frown when in disagreement, and do not speak at all. I wonder if he will start carrying his research on to your facial expressions and written notes as well. If he does, you can also take it to extreme. Having a great sense of humor, can you imagine what will happen then:
you frown, he smiles. You frown more 'severely' he laughs. You stomp your foot, he will roll on the floor with laughter...
Well, this is but one of the options, right, but now you suddenly fall on the floor laughing hard also, and it will be his turn to stopm his feet anf frown.
This kind of approach will convince him in one thing : YOU ARE THE COOLEST MOM IN THE WORLD, because YOU UNDERSTAND!
Warning: by no means, never ever do not start punishing him, do not get upset, do not try to prevent it from happening. It just won'r work for good. He is a great enthusiastic creative kid, your task is to catch up with him, and join HIS team: it will play miracles in the long run. remember: you two are friewnds for a lifetime, right?! It is not about who wins now, it is about how to be happy together.
His brain is developing, going through many stages of adjusting to this world, and he learns to cope with it, to react and act accordingly: where would he gain his experience of how to interact here, if he does not experiment. He chose you as a safe field and companion for experimentations, do not puch him aside, this is a gift: you see what is going on, you can very subtly monitor it also, retaining his trust, and seeking for the ways of guiding him, slowly but surely, to wisdom.
yes sometimes we need to sacrifice our comfort, but trust me, it is well worth it. Just, change the point of view now on this situation, and look at him as a little scientist who is exploring the socio~psychology of the world... and help him on his road.