How to build up my sons confidence

My son is 14 almost 15 and has a high pitched voice. He hate it because his older brothers is deeper and his 13 year old brothers is trying to get deep. He keeps asking when he will hit puberty and I told him he did. His voice just didn't get deeper. I tell him his voice is cute but he absolutely hates it. I'm wondering if he's been bullied for it. How can I help build his confidence?

Edit: Oh yeah he can sing really well! He just hates his voice