I wish your doc would respect you wanting to take it slow and easy to make sure it's right for your son but on the other hand, if he doesn't get a therapudic dose you may give up on it thinking it's not working when really he isn't getting enough. That might be what the pdoc is thinking.
Anyhow, only do what you are comfortable doing.
You could even nod politely take the perscription for the whole dose and then halve it at home.

Sally, Mom to Jacob 9 (Bipolar, ASD, Expressive/Receptive Language disorder, Anxiety, Tourettes-Lithium 300 mg 2xday Risperidone 2 mg a day, Ben 10 and Max 3