And there is another resason, why you should stick to the recommended dosage of the clinic: The 36mg dose most properly did the best job to improve the ability to focus and sustain attention. With the lower dose your son will not have the same benefit.
You should talk to your doctor about this issue. He might not be well informed about this new drug but it should be easy for him to get appropriate informations very soon. Maybe one of the members of the self-help group or a local pharmacists could help you and call your doctor to discuss this iussue.
If he is not willing to change his opinion you should look for a second opinion