For a 913 driver, black dot weight is 14 grams (11 grams is blue dot).
With the black dot weight your driver was about D5.5 swingweight. In my view that's high. No wonder your son had trouble swinging it.
Without any weight your driver is now about C8.5 swingweight. I believe that's more common for women, seniors and young players.
Some Taylor Made drivers had a cavity on the bottom, newer models have it filled with some kind of silicon, they were obviously...