A couple thoughts as I read through this thread:
1) Everybody in the baseball world has an opinion. If it doesn't cost you much I'd be willing to keep an open mind and listen to his evaluation. I'd also be very careful what I share. I'm a "show-me guy" (not from Missouri) who is more interested in process and outcomes than listening to somebody talk about themselves. I prefer to hear accolades from people he has previously worked with
2) At your son's age, it is about playing and developing. You want to make sure your son gets as much playing time as he can on the best team that will start him. Anything else is less than desirable.
3) Your son's height at 13 yo is going to be a challenge as far as pitching mechanics go. As the father of 3 pitchers, I would pay particular attention to what pitching advice he offers. Taller pitchers have unique challenges.
Good luck and let us know how it goes.