I agree with Kriston bad habits are hard to unlearn. There are mechanics and form involved in any instrument. Maybe the compromise is if you want a real guitar - you need a min. of x # of lessons - say 10. Then you have the basics down and can just keep working on either reading music or jamming out his own tunes.
Some instruments you can rent by the way for others interested. It makes starting less of a huge decision. We had a wonderful teacher who tossed the standard plan out the window and went at our child's pace. We like it because it is not forgiving, it does require practice, and we can't get it right on the first try. The downside at this age is it means "I'm" taking lessons to with ensure practice happens and is done well... and that part I'm not always holding up my end.
