Get that boy a fiddle! I agree that "keen to get started" is the key to learning, but I disagree that chords are the way to start.
Small hands have difficulty with chords. Especially that danged F! It's fun to play a tune that you recognize. It gives one a real feeling of accomplishment and immediate gratification
I started in second grade. God bless my folks for putting up with the first few weeks. There is something about learning at that young age that makes special brain connections. They say if you learn another language before age 8 you can be totally bilingual. Music is the best language. It may not help him speak French, but you will give your son a gift of music that will last a life time.
I switched to bass in fourth grade, but have been able to pick the fiddle back up with facility as an adult, and could do viola and cello when called for with a little practice. I still can't do chords worth a darn, but have been able to play tunes on guitar, mando and even Koto.
If he wants it giveit to him! If you can do it it's a wonderful gift to give your child. I figure if they are interested, any time after about four is a gift for a lifetime. Tell him we look forward to hearing him on Hear Me! --Beth