I was at a friends birth and we had known all along I would be her birthing partner, we had also agreed there was no way I was going to watch it come out 'down there'!
When it came to it, she was pushing and they said they could see the head, she was holding my hand and asked me 'Can you see him'?? So, still holding her hand, I looked (at this time I was pretty desperate to see what was going on tbh!)
I watched him be born, crying my eyes out, and it honestly felt like it was just me and her there, she was doing as the midwives told her but looking at me, I don't think I've ever felt that needed or connected to anyone ever (with the exception of my own children)it was amazing.
Having spoken about it lots afterwards, I can honestly say I not once thought about it being her vagina, not for a second, anymore than I ever associated my bump with my baby when I had my own.
She told me that being able to fix on my face, the look when I could see him, was what gave her the strength to push, for those reasons, I would encourage him to watch, but even if you don't want him to now, you may change your mind when it actually happens.