I was a younger competitive shooter (started in my 20s, but that's still younger) and saw a lot of transition in that time. I thought IDPA was amazing until I shot a USPSA match. then all my gear was crap. and production was super fun until I shot limited, then I got over the bullet hose stage and started getting serious about 1911s. Then I shot 3 gun, and a single stack 45 was crap for that, so got back into limited style guns....
all of this transition was over 3 years, and a lot of the gear didn't work for one division so I had to sell or trade stuff to get what I needed.
all this to say, my advice is don't spend a ton of money on gear. go to the Brian Enos forum, find a bunch of matches near you. Go early and help set up, stay late and help tear down, and ask people if your kiddo can check out or shoot their gun, see if he likes it. I bet most will. That way you can at least narrow down a style that he likes and you can go from there. Hell he may love a platform you despise, but if it fits him and he shoots it well, it will be better in the long run.
the other option in the mean time is to set up something easy that works for a lot of games. a 5" 40S&W Glock/M&P can be run stock for IDPA or Production, add extended mags for limited, add a 9mm barrel and extended mags for 3 gun or steel (or load light, get set up to load. it's not cheaper but you'll get more ammo), etc. You'll figure out which things you like and which you don't, and can focus there.
I have had easily two dozen polymer guns set up for competition over the last however many years, but if I was starting over or starting from scratch I would just get a 9mm 2011 bushing barrel with rail. I could use it (or my carry shield) in IDPA, a game I don't like. I like limited but don't care about major. I don't like open. I like 3 gun and steel challenge, and like to shoot matches at night and use my gun for HD. I like outlaw matches, and I''m too busy to load ammo anymore. I love the 1911 platform and trigger. all that said, a double stack 9mm 2011 would fit me to a T for every game I want to shoot, and buying just one of those would have saved a fortune on buying, tuning, and trying things I turned out to not like so much.