doctortran
Member
My suggestion would be (however far less credentialed than the others on here... so feel free to ignore me here) would be to learn the mental side of the game as far as a spinner is concerned (which I'm sure you already understand the importance of this anyway) rather than try and adjust his action too much until his body matures. From personal experience trying to keep your action together as you begin to grow is hard enough without trying to tinker small problems that may be resolved by increased strength anyway...
Well people shouldn't be confused about physical strength, muscle mass being related to spin and speed. In fact there is little relationship between arm strength, bicep strength, and torso strength to bowling speed. Actually flexibility is more related to bowling speed and large muscle mass really lowers flexibility. so -- don't bulk up to bowl faster.
However lever length i.e. wingspan, length of legs -- and physical speed to get through the action -- which are also major factors to speed ability are obviously significantly lower for a child than a full grown adult.