worthj1970,
Before you change anything, don’t you think its wise to consider the coaches might not be giving him the best advice? If its true that the other boy’s CU is slower but gets hit hard, that tells me whoever taught it to him did a pretty lousy job, so why would you want to listen to that oaf?
You need to assess the situation logically, not just take what clichés people throw at you, before you act, so let’s try to do that. 1st of all, let’s forget about the coach for a few minutes. Before you answer this, you need to have a firm understanding of what you believe success is for a pitcher at that age. When your son pitches, what kind of success is he having? I.e., is he getting pounded like a drum, or is he generally having average or better-than-average success?
Here’s a series of question that kinda go together.
Who calls pitches?
How many CUs does he throw a game.
What’s his overall strike percentage? (an honest estimate is ok)
What’s his strike percentage for CUs? (an honest estimate is ok)
Now that we are fairly sure about your boy, let’s change gears. How does he compare to the other pitchers on the team if you evaluate them the same way?
Now let’s try to figure out why the insistence that his CU get slower. What has been their rationalization for that? What have they done, other than throw a bunch of clichés at you to show his CU isn’t successful? Please tell me they haven’t thrown out the 10 mph difference rule of thumb, and if they have, without even having a gun to check it!
CUs don’t work because of their velocity alone. They work because for all intents and purposes, they look like a FB to the batter, and their velocity relative to the last pitch is slower. IOW, it would be kinda stupid to throw 4 CUs in a row to the same batter. Also, its velocity is “relative” to a FB, and therefore could be different depending on the FB.
FI, a player who throws 75 doesn’t need to throw a 65 mph CU for it to be effective, and a player that throws 55 doesn’t need it to be 45. It should be measured in time, not MPH. A ML player who throws 96 and who’s CU drops 10MPh has a about a 10% difference and a change of right at .05 seconds in time. But let’s look at a kid throwing the same relative velocities at 46’. That would be about 74, and to get the same time difference, the CU would have to be about only about 66. So the 10MPH rule of thumb isn’t really a good one to try to push kids into.
But be all that as it may, depending on the answers to the questions I asked earlier, why on earth would you want him to change his grip? A circle CU isn’t inherently slower than a 3 finger change, and never has been. The ball definitely moves differently, but the velocity is controlled by other things such as finger placement, finger pressure, arm velocity, amount of pronation, stride length, and many other things as well. Before you have him change grips completely, do some serious thinking! There’s nothing wrong with the circle change, but like every pitch, its not what’s best for all pitchers. 
Good luck.
