I coached pop warner for several years, and at the beginning of every season, we told every kid that if they wanted to be quarterback they could try it out. we would run them through some throwing drills, and we paid very little attention to them, then we put them under center with passing plays and a defense, after 10 reps, you know who has the mental ability to see the field and make decisions under pressure. That skill more than anything else is important for the position if you are starting from scratch, you may want to test that out before investing a lot of money and time in a QB coach, our QB was almost never the kid with the strongest arm or best athlete.
If you do get a QB coach, at the age of your son, I would try to find a local QB coach, talk to anyone at the local HS and they can point you in the right direction. Our QB's typically spent about an hour a week with a coach during the season and they were much further ahead in development than the great majority of QB's that we faced, even at regional and national tournaments, so I don't know that investing big dollars gets you much more incremental development, when he gets to HS then I would consider stepping that up.