Lefty SRH
I am a long time NRA marksmanship instructor and I commend you for helping. My recommendations would be to follow the typical shooting class outline.
1. Teach SAFETY.
2. Familiarize the equipment.
3. Check eye dominance.
4. Teach the fundamentals eyes, breathing, trigger control, sight alignment, sight picture, natural point of aim, range commands and safety again.
5. Teach each of the proper positions as you introduce them to get good bone support and stability. Remind them of the importance of follow through.
6. Don't over coach and give them too much to think about, you can make corrections as you progress.
If you are not very confident in your abilities please find him a good coach or shooting program to get involved with before he develops bad habits which will be hard to unlearn. Most young people believe what they have seen on TV and the movies and shooters know that doesn't work.
I need to cut this short because I gotta go but I will post more later.
The NRA publishes a book "The NRA Junior Rifle Handbook" which is an excellent start. they also have DVDs I get a link later.
The CMP has a Coaching Resources page start reading lots of help:
http://www.odcmp.com/Training/CoachingResources.htm
The first entry "On The Mark" is their magazine back issues are online and have many articles on teaching shooting. The second entry is a list of trainning articles by Gary Anderson (Google him)
http://www.odcmp.com/Comm/Gary_OTM.htm
Those should get you started until I can post more references.
Blacksmith