Bananaron - I owned a 17' J-Craft and grew up skiing behind a 16' J-Craft. Don't have any experience or time with or behind the new 21' J-Crafts. But I have seen these personally at boat shows and marinas.
The new J's appear to be very well built, lots of upholstery, lots of gauges and bells & whistles.
The old J-Crafts were very basic - little upholstery, a few gauges, no windshield. These didn't even have a stringers and only a 24" wide floor. The seats were on the bottom of the boat which you saw flex in bumpy water.
Slalom skiing, you could pull & tug these quite easily. I only weigh 160 lbs and I could easily tug it while slaloming. Only 1 tracking fin, 2 on the 19' J-Crafts. I cannot speak regarding the new J-Crafts.
Ironically after owning & skiing behind outboard J-Crafts and when in the market for a new boat 12 months ago - I wanted a inboard tournament ski boat. We bought a 95' Malibu Response. We love it.
Comparing my Response to our old J-Craft is like comparing apples to oranges. My Malibu rides better (slower yes), more room for passengers & gear and the swim platform is so much more convenient putting on skis. I just feel now, the overall skiing experience for skier / driver / passengers is better with our Malibu compared to when we owned our J-Crafts.
I would caution if you consider an old 17' or 19' J-Craft. I know our Canadian $$ may be tempting. But these boats haven't been produced in quite a few years - probably +15 years. The boats I have seen listed for sale here in Ontario are usually butchered and in bad shape. Decent ones hardly go up for sale. The main reason I went to a Malibu.
Good for you & your son MotoXR if you found a gem! Congrats!
Good Luck Bananaron with whatever you end up with.
Hutch