OK OK I can't take it any more -I confess: When my son was twelve I lost faith in the local coaching. I was convinced (and rightly) that while most of them played reasonably well, they didn't really know what they were doing and had lots in their game that was idiosyncratic. Fortunately, there was a very good coach in town for a few months, Joe Brandi. I took the kid to him and he started the kid on a OHBH. He was very patient, soft-spoken, and effective. At one point he was working on the grip and (integral to that) the angle of the racquet handle to the forearm. I actually said, and this is embarrassing, "doesn't Pete hold it with much more angle to his forearm?" Joe just looked at me and softly said "yeah, but not until he'd hit 10,000 backhands holding it the way I'm introducing it." Well, I never said another word...
Three or four years later I took the kid to Smith/Stearns for a few weeks, our first father/son road trip. When I wasn't hitting with other people, I'd watch the kid and the others from the bleachers of the stadium court, looking over toward the clinic courts. Never said a word. Smith did more for the kid's serve in a short private bit than other coaches had done in many hours. Pick a suitable coach on good information. Let him (or her) do his thing.