Concerta is a rather new psychostimulant version, containing the known methlyphenidate. But it lasts much longer: between 10 and 12 hours due to a specific transport system (OROS). If your son would take the medicine at noon he might get severe sleeping problems.. That is why a single dose in the morning is recommemded.

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.