I am 14 now and I pitched at 60 feet for my select team and 54 feet on my middle school team last year. It was a big mistake for me to switch back and forth. I was very effective at 54 feet but I wasn't at 60. At 60 feet after pitching at 54, everything would be in the dirt. But after pitching at 60 then going to 54, I could get alot of swing and misses on high fastballs. This only worked because I could overpower the kids and I got away with a mistake every pitch.
I would stick with one, but don't workout at 54 in the offseason and go pitch at 60 in the competitive spring and visa-versa. My advice would be to pitch at 60 so that he is used to it going into high school. I don't want to state the obvious, but you want to make the best FIRST impression for your high school coach as possible.