jd is spot on...I saw his short-and-to-the-point remarks only after I had finished the long-winded diatribe below...
If you also have become pretty knowledgeable about pitching mechanics over the past 4 years, and if you completely buy into what your son has been learning from his long-time pitching coach, then the strategy for dealing with a new coach who wants to change him needs to be carefully thought out.
Prime examples of the usual kinds of well-meaning but misguided changes that poor coaches may push on your kid (1) immediately change the arm-slot of a pitcher who doesn't fit the coach's myopic lens (this is almost always: get your elbow up and throw over the top, son), (2) ask the pitcher to "stop at the top" of leg lift, (3) tell the pitcher to pull his glove into the body during launch of the ball.
There are lots of other common wisdoms that do not bear close scrutiny, but pitifully few coaches at LL level, 12U travel ball, etc, are really curious enough or motivated enough to learn pitching in any depth. So, they tend to accept common wisdoms uncritically and teach the same things that other ignorant but well-meaning coaches have taught them in the past.
After 4 years with a really good pitching coach, your son should have enough confidence in most cases to look his new coach in the eye, smile sincerely, and say "thanks for that advice, coach" or "thanks, coach, I'll try that" and then quietly keep doing the things that make him a good pitcher. Most often, since a coach's time at your son's level is spread thin among a lot of players and the coach will only remember that your kid is pleasant and cooperative. If your kid is good at what he does, the pressure to change will likely abate.
If the new coach is a red-*** about your son following his advice, then he (your boy) will need to take a different approach. If it's "change your arm-slot to ___", I suggest that your son look the coach in the eye and say, "Coach, when I try to throw like that it makes my elbow/shoulder/etc sore". This is not so much an untruth as it is a truth told out of sequence. If your boy has adapted his throwing musculature to a certain arm-action and arm-slot over the past 4 years, changing that abruptly is a poor idea.
Anyway, you get the idea. If you and your son strongly believe that his old pitching coach was doing things right, then your boy needs to develop the confidence to stay on the track that you and he think is right.
If you don't have that confidence after 4 years with the same person, then you do need to think more critically about what both of these coaches are telling your son.