Although your son may believe his present teacher to be a good one and although he may be active socially and athletically within the confines of this very small parochial school, I'm guessing he has felt before and especially now (at this emerging adolescent age and stage) that he is missing out on the much richer and stimulating life of a public school. What would the harm be in allowing him to enter a public school and then assess which one suited him best? Unless it would pose an economic hardship for your family, I can't understand why experiencing the life he presently desires would be anything but positive for him.
He'll be able to compare both academic/social worlds and you will be granting him an opportunity to assume some appropriate responsibility for how he wants to be schooled. He can always return to his old school, can't he, if the public school doesn't prove to his liking?