Sorry the dx has been such a shock, I know how you feel, I went through exactly the same when DS was diagnosed last year. I came very close to seeking a second opinion, but I finally gave up as I didn't want DS to go through any more assessments.
Be kind to yourself, allow sometime for the dx to sink in. Come to this board for fantastic support. I would recommend Ambitious About Autism forum as well.
What helped me the most was reading about ASD, attending courses and try to become an expert on DS' needs. We are a much happier family as a result.
We've been lucky the LEA has just agreed to carry out Statutory Assessment of DS' educational needs. We found the Autism Advisory Teacher's support essential in achieving this as the SENCO had been reluctant to apply pre-dx. But you could apply for it yourself, no need to wait for the school to do it.
If you decide to apply for DLA check the CEREBRA guide , is very good.
Maybe is worth asking for Occupational Therapy input, DS' paed referred him to the service.
You might find meeting and talking to other parents in similar circumstances useful so check your local NAS for a carer's group.