16u FB and CU w/ little speed separation

My son is 16u LHP 6’2" 165 lbs. His FB and change-up are consistently only 2-3 MPH difference. The CU is considered his best pitch by most that see it (75-76 MPH). 4 seam FB sits 78-79 MPH. He’s thrown the FB at 83 max in BP session a few months ago. Pulldown is at 91 MPH. He’s tried gripping fastball more w/ fingers but same 78-79 velocity. Any ideas on why he’s not getting more separation between FB and CU? Thanks,
RG

My son had the exact same problem. He tried multiple grips. Tried curling his toes. Tried a stiff wrist as if he was pulling a window shade. In all instances the difference was about 2-4 MPH from his fast ball. After several years of trying and now pitching in college, he is throw a split finger. Is getting a 8-10 mph difference between the two.

Perhaps the solution is to put more skin on the changeup or grip it more tightly. The additional friction might slow it down a bit more.

That said, if the changeup is getting good dropping movement, perhaps it doesn’t matter. Stephen Strasburg’s best pitch was his power changeup averaging 87 mph. That pitch practically dropped of the table. Throwing it hard is what made it so tricky.