There is a wealth of information out there to be had. It doesn't take much in the way of Google-fu to come up with the Autism Society of America
or the National Autism Association
Both of those are good places to start.
For your specific situation; I have a few things to consider, bits of advice:
1) It's called a spectrum for a reason. A child with autism may be only mildly affected or severely affected. Also; it's quite common for a child to have some traits or measured levels that qualify as autistic, while they may be normal or even above normal in other ways. Each kid is different. Thus, they may be considered severely affected on one axis, moderately so on another and totally fine on one or more others. This may explain why your sons teachers think he is fine. The attributes they see are within the normal range, whereas you see the bigger picture.
2) Proper testing and diagnosis starts with your family physician or pediatrician. You need to set up a 1/2 hr appt with him or her and explain why you think your child has an issue. Your doctor will then hopefully refer you and your child to a specialist in developmental disorders
2) Ultimately, autism is a diagnosis of exclusion. An average child goes through a year or two of testing before a diagnosis of autism is reached, because there are a large number of other learning disabilities or developmental disorders that share similar symptoms with autism. You need to prepare yourself to be patient during this phase.
3) There is no medication, no regime of diet or therapy is that is a "cure" for autism. (indeed, a large segment of the autistic community argues there is no *need* for a cure, they are fine the way they are...) What exists are teaching and coaching programs to help the child consciously master the soft skills the neurotypical majority take for granted. There are medications which can help buffer the secondary symptoms (anxiety, depression, co-morbid sleep disorders etc) but for the most part it is the patient teaching that creates the change, not the meds.
4) The teaching and coaching isn't just for the kids. Where I live, there is an excellent program (under the auspices of local community mental health groups) to teach the parents, and in some cases, the siblings how to understand and interact with the affected child. If such a program exists in your area, I highly recommend looking into it for your whole family.
5) You will constantly encounter people who have formed opinions about autistic kids based on incomplete or outdated information. The withdrawn rocking child is the archetypical face of autism, but one that only describes the most severely affected. (and, as described elsewhere in this thread, a symptom of an autistic kid who is overwhelmed, hasn't been given the mental tools to cope with excessive stimuli.) It is possible that your child's teacher has the assumption that all autistic kids are that withdrawn and unsocial archetype. And let us not forget what the other gentleman (ourlovecanlastforeve 795111) above blasted. There are many many people who think the misbehaviours, weird obsessions and compulsions are the product of bad parenting and they are quick to judge you accordingly.