Boyfriend grows mid sex??

So my boyfriend told me before we got together that he could range from 6” erect to 9” erect. It depends on how horny he is in the moment, I thought he was lying and then when we got together he would actually grow while inside me during sex! It was like I was having sex with a different person every time. I have never had a guy actually do this during sex, it’s usually just the same size until they are done and shrink again. I was curious if anyone has ever had this happen to them?