My son has mild to moderate autism, he recieves occupational, speech and behavioural intervention, we have never has to use meds with him, the intervention therapies alone have made a huge difference to my son's life.
It is at all possible you could get a referal from your son's doctor for intervention therapy?
I believe that just medication alone is not regarded as helpful as a combination of medication and behavioral therapy.
What does the doctor say about another medication? Have you asked?
Is your son on the medications for behavioural issues? Does he recieve any types of intervention therapies?