I personally think that many, many children do this in regards to the negative self talk.
And they may do it for different reasons.
One of those reasons is that they may be thinking, "I know I shouldn't have done that, but I did it anyway, therefore, I must be dumb."
OR, if the sneaky little boogers can get you to feel badly for them, maybe they won't get punished after all. Hey, when you're a kid, it's always worth a try. If this tactic doesn't work, they can also try the, "But remember how cute I am? How could you not love this little face?" tactic.
Just make sure you differentiate between the child being bad and the behavior being bad. They really are two different things and 6 year olds can easily understand this. "I love you, but I don't like the disobedience, at all."
Both of my children tried the "nobody likes me" routine and you know what I did?
I started singing, "Nobody likes me, everybody hates me, I'm going to eat some worms. Short fat fuzzy one, long slick slimy ones......"
My kids could never keep a straight face so I had interjected some humor long enough for them to quit their pouting and talk to them about why they were in trouble and why pouting wasn't going to get them out of it. Their punishment was still the same, but it's because I loved them that they weren't getting out of it.
Love, hugs, and kisses, but there was still going to be a consequence and if they didn't want to be in trouble for the same thing again, (and here comes something my kids could say in their sleep)....
"Think before you do."
Just be consistent with your son. It's better to get all of this stuff out of the way now as opposed to later.
Best wishes.