Sailing with other boats such as at a dingy school/club is more fun than one boat on its own, but not always practical/possible. I would however strongly recommend
learning in a
dinghy before setting foot on a keelboat. Dinghies will teach you more about sailing in an hour than a keelboat could in a week, have lower loads, and are much less expensive. Even a larger dinghy like an older Wayfarer/CL or similar can be a lot of fun while also being much safer and easier than a performance boat. I have sailed (helmed and crewed) Optimists, Toppers, and Flying Juniors as a kid, a Tasar, a Wayfarer, a
Ranger 22, a Sonar, an Elliott 770, a Yamaha 25, numerous Santana 525s, an Andrews 26, a Dehler 29, and a J133. Out of all of those boats I would have to say that the Flying Juniors and Tasar have taught me the most about sailing, skills which I have been able to transfer to keelboat sailing and
racing. As a side note, good on you for doing this for both yourself and your kid. As someone whose parents got them into sailing at around that age, I believe that the best way to start would be to put him into a week or so of
sailing lessons on small boats. The instructors are knowledgable and know how to
work with the
kids on the natural fears associated with the sport. Once you come to grips with the fact that the worst thing that can possibly happen is you getting wet and having to right the boat, the sport becomes a lot more enjoyable. I regularly
capsize my dinghy now for fun or when it is too hot and the
wind is lacking. Plus I remember that at my dinghy
school it was a running competition to see who could recover from a
capsize the fastest which adds a lot of fun to learning a necessary skill. Competition is always a good way to help build skills.