Should I get a second opinion on my son's PDD-NOS Diagnosis?

My son had all the "clic" symptoms of Autism as a child 9 mo-4y/o. He was considered "tered skilled" and "developmentally delayed". He had major communication delays. At about 4y/o my son began "waking" up, so to speak... his vocabulary was still low but, he would attempt to communicate. Thanks to our pedi, my son had early intervention that has helped him tremendously!! We still had communication and social issues so I decided to have my son officially tested. He was diagnosed as PDD-NOS (on the spectrum but high functioning). My son is currenty 6 and scores with a high IQ and can read and more advance than some of his peers, But he still has communication and social issues. He speaks clearly and if you're patient you can understand what my son is trying to convey, but he doesn't speak in full or complete sentences. His social skills are getting some what better due to curiousity but he still opts to play alone or not engage in peer play at all. We got a diagnosis, but I'm wondering if I should get a second opinion. His Dr. said if he didn't read he would have definately fell as a firm ASD diagnosis. I spent years helping my son, going to therapies every other day, reading books, educating myself on how to better help him. We've gotten this far but we still have issues to work on. Therapies are very expensive and the cles I want him to take are way out of my budget, I want to get a ASD diagnosis so I can get better insight on resources and possibly get more therapies as he gets older. I definately think we can knock this but I need more time and I have no idea where to go, where to look. I can handle helping him understand elementary aged curiculum but as he gets into higher grades I'm afraid I won't be able to help him. I'm stuck on what to do? Should I try for a second diagnosis...if I do, do I tell them he has already been diagnosed? He's such a great little guy I just want him to have a chance... he doesn't know anything is wrong, all the "check ups" were just to make sure he was growing up strong and healthy.