For a 2015 I would not be too overly concerned about what the radar gun (or pocket radar gun) is saying. I assume you are asking about reliability of this product. It varies, and since most pro scouts use stalker guns, that is the gun I would most trust. Also, you can get different readings on guns, depending on the angle the reading is taken from
More important at you son's age is does he get people out? Good command? Working on an effective change-up. Can he throw his fastball for a strike?
Lots of time for him to still grow and work on his velocity. Enjoy this time and let him have fun. It all goes by so quickly.