Since he has been diagnosed with this 3 times I'm wondering if you brought up your doubts and concerns when he was diagnosed the 2nd and 3rd time? Because you're not a doctor you need to trust them, but it certainly doesn't mean you can't question them. You are your son's number 1 advocate, you need to be the one to fight for him to help him as best he can be helped.
Remember that not all children will exhibit the same signs, the two main characteristics of the disorder are difficulties with social interaction skills and communication. Treatment is a very intensive, comprehensive undertaking that involves the child's entire family and a team of professionals. So my advice to you is to 1) see how the Abilify works on his aggression, the prescribing doctor will need to know specifically, 2) and to become an expert on PPD-NOS, read everything you can, keep a journal with how his day is going, his behaviors, and questions as they arise, and talk to other parents of children with the same diagnosis, so that when you deal with doctors, therapists, etc. you are armed with sound information and everyone can be on the same page.
Here's some information to get you started:
http://en.wikipedia.org/wiki/PDD-NOS
http://www.autismspeaks.org/what-autism/pdd-nos
http://www.nationalautismresources.com/autismsymptoms.html
http://www.autism-help.org/pdd-nos-overview-autism.htm
And here is "Cutting Through The Psychobabble:
Diagnosing Autism And PDD-NOS Per The DSM-IV In Layman's Terms"
http://www.bbbautism.com/diagnostics_psychobabble.htm
It will hopefully help you understand how your son's diagnosis was made.
Juanita, find a support groups for parents whose children have the same diagnosis as your son, in a group setting or online. No one expects you to deal with this all on your own and you don't have to. There are others who are where you're at or have been there and can help.