I don’t believe he’s at all stupid. Rather, he’s well on his way to becoming a pathological liar. I have one in my family and he acts the same way. After telling a huge one he looks around and says, “Who me? I would never say anything like that!”
I got one of those in my family, too. It's a sign of narcisstic personality disorder...read up on it. Their motto is "all for me and all for free."