I agree with the previous posters--I wouldn't worry too much unless it is really interfering with his daily living. I also think that a child under 3 really isn't expected to be social with other children especially if they have interests that wouldn't be the same interests of a normal 3 year old.

Keep us posted. I'd love to know how his math/number skills are progressing since DS5 loves his numbers as well--just not to the same degree.

Crisc