I am 52 years old and I just started learning the violin last year. I also have high functioning autism. Autistic people are visual thinkers. We like to see things right in front of us to learn. So in addition to the recommended structured lessons and practice, I would suggest videos demonstrating the different topics brought up in the lessons, because telling an autistic person something and showing them something are two separate things. Also books. If he chooses songs either from the music store or library to play in addition to his lesson, he will be less likely to quit the violin. I also suggest videos, such as youtube(and violin tutor pro), where he can watch others play the violin and see things visually.