I think sometimes it can be hard to tell the difference in an early bloomer vs. being gifted. Either way, your son is very bright!
Like the previous poster said, gifted kids can be quirky. I have an almost 4 year old who scored very high in testing yet she did not walk until 14.5 months. I think for me, the biggest indicators outside of the test scores, has been DD teaching herself to read and write, her obsessions with subjects not typical for her age (like anatomy) and just her general behavior. She's a very intense child!
