Texan -- I'm not indicting you here -- but have you ever asked a group of kids to describe how their arms feel -- especially in a group setting? I'm sure you have a great handle on your son and how he communicates....but "tired", "sore", etc. don't always mean the same thing to the same kids. We had one kid that described his arm as "spaghetti" --- can't remember if that was good or bad, but it sounds bad to me.
I agree with the stance that no coach should tell a kid to throw through "pain". Discomfort caused by early use may be a little different -- kids usually don't develop tendinitis during the first few days of "normal" throwing (I don't think) -- unless they jump in the bullpen and try to light up the radar gun after not throwing for 2 months. A coach should be specific in determining the types of pain --- elbow, shoulder, front, back, etc. -- then be careful with things. But...if they don't know (kids won't always "confess" to discomfort) then this type of thing can go undetected....especially when a coach is evaluating 45 kids in a short period.
JMO.