I assume you mean high school coach where there isn't an option to change (i.e., as in travel).
Teach him that it's the reality of the world. He'll have co-workers, bosses, subordinates, etc., who aren't necessarily the most knowledgeable throughout his life. Be respectful and change the things you can and tolerate those you cannot.
So long as personal safety isn't a risk of the result, then he should put forth 100% to try and do what the coach asks of him.
We've had some similar situations in our program where there are some fundamentals being taught that are not necessarily the best way to do something. Nothing that would injure someone, but simply less efficient or different. I've told my sons that when you're playing with coach A (HS), do it how he says, when you're with coach B (travel) do it the other way.
Actually had a scenario where one coach taught the pitchers how to catch the ball being returned by the catcher. First week of summer after HS, my son was catching the ball that way. His travel coach asked him what the hell he was doing. Son told him that's how they have to do it on the HS team. Travel coach's response was "stop doing that, you look like an idiot".
In all cases, be respectful and mindful of whose field he's on and behave accordingly.
I must misunderstand this. Surely you are not in favor of constantly changing mechanics to appease various coaches?
If you read my post, I clearly qualify fundamentals and ensuring that safety (not putting player at greater risk or injury). To that point, I'm going to get very specific. You state "constantly changing". I don't know what you mean. If you believe having a spring coach tell you to do it one way, and a summer coach another, then we're talking about doing something two different ways a year. I don't think that's constant, but perhaps you do. I'm assuming that we're not talking about playing for two different coaches simultaneously (coach A and his way on M,W,F and coach B on Sat, Sun).
With respect to changing mechanics, yes, why not? When players see a different pitching or hitting coach, they typically have different philosophies and will tweak different things. This is really no different. Most HS coaches teach a certain way to their beliefs and travel coaches may teach another. In trying different things, you can often find something that works better. I certainly subscribe to the "if it ain't broke, don't fix it", so if it's working, leave it alone. Also, if the kid is perfect in all facets and no improvement is necessary, then NO ONE should be changing anything. My guess is that kid isn't out there this season.
Finally, to be frank, I'll give two specific examples of what I'm talking about. One coach teaches how to crow hop. He has these kids basically performing an Olympic quality high jump when throwing the ball from the outfield. In my opinion, it's inefficient and simply looks ridiculous. They perform drills where they crow hop over a bucket to practice throws in from the outfield. It's the most exaggerated crow hop you'll ever see. Guess what, that's what my kid does when he's with that team. Not the other. Other example. One coach expects the pitcher to receive all throws back with two hands in a specific catching motion on every throw back from the catcher. When playing with that team, he does it. Not with the other. Both of these are rather benign, but it speaks to my point.
In both examples, he's respectful of the coach with which he disagrees. He does it his way, then doesn't with the other coach. Neither puts him at risk of injury. I do believe one is less productive in producing results, but that's the coach's wish, so it's done that way.
EDIT: And apologies for such an anal reply. I've been working on taxes all day so I'm in "that mode"
I thought it was intentional.