I usually say "on your left" loudly as soon as I know I am barely in hearing range. Most people can't make out what I said but they hear me say something and I am far enugh away that they look and have time to think, "Ohh its another guy who needs to pass" and they get over. If there are multiple people talking or I know they won't hear me, I usually just pass silently on their left if there is sufficient room....or if not enough room, I just slow down and tailgate them until I can get around.
I used to do what many regard as the proper technique and slow down, wait until I was close enough to be fully heard, and then announce in a loud clear voice, "on your left." However, when i did that I had a couple occasions when it caused the people to whom I was speaking to panic and dodge in front of me or jump off the trail and make us both feel uncomfortable about their overreaction. After a woman with a dog did one such panic and jump off the path maneuver and almost wrecked me, I switched to the alternative technique described in the first paragraph.
As for the OPs situation, i am not sure I am ready to blame him completely if it happened like he said. I mean, you should always give yourself sufficient room to react but hey, I should eat more leafy greens too. I would chalk his wreck up to "S&%@ Happens". Sounds like you and the other kid both learned a lesson. That idiot will now know how to use a bike path and you will know to watch out for idiots.