Slotty, I can't tell you how many times I have heard my son say - they can't catch up to my fastball, why should I throw the change? I am in a similar boat as you. HS Junior, fastball right now usually 85-88 in game, wicked slider. I keep pushing son to continue to work with change. He is getting better at using it. When he does it is pretty darned good as well.
I would say when a batter starts to catch up to your fastball is a good time to use it. It's also not a bad time to throw it off the plate on a 0-2, 1-2 count, just to throw a batter's timing off. Let him see something slower. It will make the FB all that much more effective. He has thrown good ones right down the pipe with 2 strikes. When the pitch is working, it drops off a table around 8-9 mph below fastball speed.
Bottom line is, I personally believe that even though your kid is a hard thrower, you still need to work on the change. It is a very valuable pitch to have in your arsenal.