Hi Weisan,
We also have a son who is on the spectrum (high-functioning autistic). A developmental pediatrician is a great suggestion. Another possibility is a pediatric neurologist who, if you're lucky, specializes in this stuff. A word to the wise: neither of these docs grow on trees as it were. I live in Central NJ and was willing to travel to NYC or Philly to find what we needed. Be prepared to wait months and hope that you desired doc is in-plan because it can get costly in a hurry.
I'd also recommend you take Jason's advice - don't wait for a diagnosis. Start with a school counselor or case worker. Also, Applied Behavioral Therapy centers are all the rage now. My insurance has has only been covering ABT for about a year. We have someone coming to our home 3 days weekly and it's been a godsend.
I'm sure with the proper treatment, patience, and lots o' love your son will thrive. My cousin's son has Asperger's and will graduate with a Doctorate of Music degree from Rutgers in May.
I wish you much success on your journey.
If I may, please indulge this proud papa:
https://www.youtube.com/watch?v=E36dTuueY5U