Sometimes.... a child self-talks that way.... because... that is what they are told... by the Parent.
When I was young... my Mom was very critical towards me and my siblings. It then becomes something where the child will actually 'believe' they are... what the parent tells them... because it is repeated to them, over time.
For a long time, I too... believed I was "stupid." (although I was not).
It took me a long time... to overcome. Because, when a Parent uses words like this, against their child.... it damages them... their soul, their emotions, their mentality, their being, their sense of self.
So... I would be very cautious... because.... that boy, your Boyfriend's Son.... is to me... repeating... only what is told to him and how he is being treated. For which, you do not really KNOW... how your Boyfriend treats... his son.... in private. You have only been seeing this Man for 6 months. And because... you do not really know... how he parents... his son.
Personally, I have had many times... when I feel I have 'clicked' with a guy. But later... learned... that that guy... was just a nightmare. In the beginning... they always.... seem more than what they actually are.
So... be cautious. Especially... since his son is displaying some emotion based... issues.
But still, it is not your place... to step in.
all the best,
Susan