I recommend Freeing Our Families from Perfectionism by Thomas Greenspon.

DS6 is perfectionist. He too used to refuse trying new activities, but things have improved greatly in last year. Now we go weeks without even thinking about it. For what it's worth his perfectionism peaked a few months after starting K in Montessori. Once we started homeschooling he learned that there is such a thing as good enough and it's fine to miss a question here and there or even a few in a row!

Good luck

LMom