madeinuk, Thanks for sharing, it is the first time i heard about "synaesthesia"

After reading all of your reply, i finally understand that why the psychologist said that she can't rule out ASD for my son and have to have him evaluate every year. Before age 3 it is really hard to see how "social" they are, and most of them may only like to talk about sth that they are interested in. I will keep in eye on his social development and try to help him in this area as much as possible. Thanks all ^^