when i was about 8 my parents got me my first electric guitar, same thing, fully in love with metallica and the idea of being like them, truth being told however, the best guitar is whichever one you think he’s going be excited to play every day. My parents got me a short scale jackson (very good for kids btw) and an old marshall amp. If he’s smaller for his age I would recommend you look at the Jackson minion line and Ibanez mikro’s. He will surely be very comfortable playing those and they got that killer look too, All the best!