Why is he SO insecure ..... ?

My boyfriend and I have been dating for 11Months. (We're 17) To make it a long story short , hes plainly just insecure. & I don't understand it . He's not the nerdy type . He the star basketball player , hes the Quarterback for our football team , hes one of those y type Jock boys & hes EXTREMELY cute & A LOT of girls would love to date him. But behind closed doors hes one of the most insecure people i've ever met. His life is pretty decent except for the fact that his dad pushes him to do everything more than the normal parent should. He complains a lot of his imperfections & usually i'm the one to encourage him. In front of people he acts like hes the **** & he knows hes the **** , but like I said behind the scene's its a completely different story. I was the first person he lost his Virginity to & he was still really shy before that , but I thought it improved but idk I guess it didn't . I don't nessacarily know what his problem is .. I think it's thinks with his family ,( dad , twin brother (fraternal) , or mom idk ) but I don't wanna make him uncomfortable in asking . But I really don't know what the problem is , but I really care about him & I want to know? Helppppppppppppppppppppppppppppppppppppp… .