I didn't read every word of every post here, but from what I've read, there is a lot of good advice from people who appear grounded and understand the big picture. So I'll just add a little bit.
IMO, from what I've seen as a high school coach, the easiest thing to correct is defensive fundamentals. For this, you can get the basic fundamentals from YouTube, have your son watch and understand the videos, and just rep the crap out of it. The sooner he can master the fundamentals, the sooner he can consider more advanced defensive lessons. But really, I don't think that's necessary. I'm afraid taking defensive, position-specific lessons too soon would pigeonhole your son into certain position(s) too early on. Just rep it out. And if he gets to middle school and his likely positions begin to become clear, advanced lessons are fine if you think they are necessary.
Second easiest to correct would be tweaking someone's swing. Notice I said "tweaking," not teaching. Certain mechanics are harder to change than others. For example, learning how to use one's lower body properly, and maximizing the kinetic chain within one's swing, are tougher to teach than bat path and upper body mechanics. They're still fixable, but not as easily.
With that said, most people spend there money on the swing. But be careful - I know many "hitting coaches" who simply make money using their facility for BP. No teaching, no explaining, no videos, no feedback... just a little tee work, a turf cage, and a fancy pitching machine. If they are taking lessons at a young age, they need to be primarily focused on learning the different aspects of the swing. Balance. Purpose of the stride. Lower half mechanics. Swing sequence: Hips --> shoulders --> arms --> hands. Palm-up palm-down through the zone. How not to drop the shoulder, etc. And LOTS of tee work. So if you are going to spend money here, don't keep sending your son if you're not getting these things.
And with hitting, there are hundreds of hitting gurus with hundreds of different techniques and philosophies. It really all boils down to the same stuff in the end, while also understanding that every batter is different. Anyone promising to magically create the perfect swing for every batter is lying.
The third hardest thing to teach is baseball instincts ("baseball IQ"). Whether it's baserunning, remaining calm and knowing what to do under pressure on defense, or understanding your approach at the plate, the best way to build baseball instincts and IQ is just to play. A lot. While I am a big supporter of local kids playing rec ball with their local community team (albeit against travel tournament competition), those teams and leagues normally don't play many games. The more games you can play in those middle elementary grades, the better (to an extent, obviously). HOWEVER, this only applies if you have coaches who treat the boys' arms properly. If you have the Johnny All-Star Coach throwing the same 2 pitchers every game and not moving the players around defensively, find another team. And I'm also an avid proponent of playing as many sports as possible, so by "the more games, the better," I just meant during the summer. Boys' arms need a break. If they don't play a fall sport, that's a great time for rest and to hit the weight room (or do something else, anything else).
The hardest thing to correct is someone's throwing motion. If you were ever going to spend money on anything, I'd spend money getting good help to teach your son how to throw properly. Breaking hands apart at the belly button, power position, arm path, using the wrist, finishing over the opposite knee, tucking glove into chest/armpit, etc. I've coached 15+ years combined at high school and 14U, and you'd be amazed how many middle schoolers cannot throw properly. And by that age, it is really, really, really difficult to correct how someone throws. This doesn't have to be pitching lessons - just playing catch properly. 'Cuz let's be honest, if you can't throw, you're going to be stuck without a position unless you're an exceptionally rare athlete who can rake.
Sorry, I ended up typing more than I planned. Just my $0.02