Not posting as parent of special needs here, but as older sister of special needs.
My brother has been given every diagnosis possible...literally from completely normal/parents stink to mentally retarded, Autism, Aspergers, and a variety of other diagnoses. And the vast majority of them have been changed. For us, ASD doesn't stand for Autism Spectrum Disorder, they stand for "Alphabet Soup Disorder", because that's what my brother has. If it has initials, he's either been diagnosed with it at some point, or they think he really has it still.
Keep in mind that my brother is 22 years old. When my parents first started on this very long journey, there early intervention programs didn't exist. They relied on their ped (who was very big into blaming everything on our parents...so he hasn't slept for 4 days straight? That's your fault!). They finally (when he was 3 or 4) met with a child psychologist who agreed that he had problems that weren't parent caused. First it was ADD, then ADHD, then they added and subtracted disorders for the next 15 years or so.
He still has no "formal diagnosis" beyond ODD, RAD, mild MR, mild DD, some speech issues, OCD, and a disorder causing him to have violent tendencies toward appropriate authority figures and select inappropriate authority figures for himself...I just can't remember the initials and don't have time to pull out my paperwork.
My parents are so jealous of parents today...there are so many intervention programs available that just weren't there 20 years ago. Today, my 22 yo brother has the physical capabilities of a 12yo and the mental capacity of an 8 yo. And to this day, we don't exactly know "why". The best we can do is that he was 3mos premature, weighed 1lb, 15oz, and had a brain hemorrage at birth.
If you think something isn't right...keep trying to figure it out. Get a second opinion, and then get a third. Keep looking. While it is possible to grow into an adult without a formal diagnosis, it's definately the more difficult path to travel.