Hi fish10
Before getting into which dive ops to go with, I'd like to point out that with just about all of the resort islands, there will be only one resort/hotel on each island. That's because most "resort islands" are essentially large sandbanks covered in vegetation, and this restriction in real estate size determines the number of rooms/guests each island can accommodate. Thus you will find only one dive op per island (= choice of resort automatically determines the dive op) except in Male, the overpopulated capital island.
As for diving in small groups, I'd say it heavily depends on which resort you stay in. Generally speaking, resorts with more rooms/villas will have more divers, resulting in under water group tourism, while resorts with less rooms/villas will have less crammed dives.
My experience with Sub Aqua in Hilton Rangali (150 villas, now Conrad Rangali) in my view was the former, diving in large groups overseen by a number of dive masters. The guiding was efficient and service was superb. The dive dhoni was large and spacious too, so I wouldn't call this "cattle class diving", but it would be a lie to call this a small group diving.
Then, Soleni Dive School in Soneva Fushi (65 villas) was the latter. On a typical dive I had, there were 4~5 guests diving with one dive master + instructor, while the dhoni was manned by captain + two deckhands. Definitely a small group, but seasonality may have had sway on how (un)busy things were.
That said, I think you should first narrow down to a couple of resorts you may wish to visit, and then contact their dive op about their seasonality and busyness. I hope your dives will be memorable, and "the dive of a lifetime", especially for your son.
Regards
Kaza