Thank you to cammom and epoh for your responses.

DS took the WPPSI-III and he scored:

Verbal 127 (96% percentile)
Performance 148 (99.1 percentile)
Full scale 139 (99.5 percentile)

I think the scores place him in the moderately gifted to highly gifted category.

We (the psychologist who administered the test and me and my husband) think he didn't do as well on the verbal subtests because he speaks predominately mandarin Chinese and he attends a bilingual preschool (English and Chinese). He's raised trilingual: Mandarin Chinese, Taiwanese and English. He speaks in 1/2 Chinese and 1/2 English in a given sentence. We are thinking of having him retested in 1-2 years to see if the scores change.

The reason why we had him tested at such a young age was because his preschool teacher suggested we have him tested. He started writing at 1 year 10 months old, reading at 2 years old and doing simple additions and multiplications at 2 1/2 years old. He has a crazy imagination and started arguing with me when he started talking at 1 year 3 months old. He plays board games fitted for 6-8 years old and can play poker by age 3. Also, when he was around 2, I was describing him to my family therapist trying to get some parenting tips because he's highly sensitive, and my therapist suggested to me that he may be gifted. To be honest, I am slightly embarrassed to say that I find the news distressing. Am I the only parent who feels that way? I don't know how to raise a gifted child. He is very intense and actually became more mature when he turned 3. However, now that he's 4, he has regressed a bit and I think it may have something to do with us having a new baby in the family.

Anyways, I am trying to get help and support by participating in forums like this one and educate myself by reading books on gifted children.

I will take your advise and visit the schools to see which environment is the best for him. I guess I am afraid he will stagnant and not reach his full potential if I don't find the right school for him.