I was at my mums birth (i was 12) when she had my little brother.....to be honest i didnt really understand fully what was going on (i thought the umbilical cord slithered back in!!

) I remember the head coming out and to be honest, i never really saw it as "my mum's vagina"...it just was what it was....and it gave me and my little brother an extra bond and we are very close now and i think that is because i saw him being born....
All i would say is make sure he is aware that you will be in a LOT of pain but that you will be ok....it freaked me out a bit that my mum was in soo much pain....