Don't beat yourself up over this. I know it's frustrating. I had a similar experience last season with a small 5 point. I took a quick shot right before dark, not exactly the best angle. I shot at it with my muzzleloader and couldn't see which way he ran because of the low light and the cloud from my muzzleloader. I found blood and hair but then realized that it was from a doe I had shot earlier that day in the same area. We looked for about 45 minutes that night and decided to try again the next morning. I didn't get a lot of sleep that night! About 6 of us looked for him thae next day. After about an hour and a half of not finding a single sign of hair or blood my friends had given up. I circled back around one more time to look. Every couple steps I did a sweep back and forth looking for him. For some reason I looked over my shoulder and I spotted white belly hair, I finally found him! He was about 150 yards from where I took the shot. I was lucky to find him because there was little if any blood or hair to track.
Anyway keep looking. I've helped friends 3 or 4 times track deer that we never could find. Don't be afraid to call deer search. Guys I've talked to in my area said they are very helpful.
My hats off to you for taking a young hunter under your wing to introduce him to bowhunting. I think that's great! This can be a little learrning experience for him.
Good luck. I hope you find him. I'd like to see a picture of that 8 point soon.

Mike