The entire year our DS was 2 was exactly like this, but first it was letters until he mastered them, then is was anatomy, space, etc. He started numbers at 3 and counts everything right now. Just tonight, he typed out the alphabet in order and then in patterns on the laptop before bed and then counted the letters for fun, he said.
Letters became characters to him. He actually drew them as cartoons and gave them personalities and names, they had families, the upper case being parents and the lower case being children, of course.
He explored letters beyond anything I could imagine, he would sort them by the angles of their lines toward 3 when there was nothing left to explore. When he was 18 months to 2.5, I was concerned that he might be on the spectrum, but it was ruled out (several times). He's slowing down on obsessive sessions or they just seem less intense. But, he is very emotionally intense, still doesn't like transitions and is calmed by his academic interests. We talk about it now and he is starting to understand and be able to talk through transitions.
All that being said, continue to talk to the doctor about your concerns. In our case, the doctor and child psychiatrist both said that he had so many traits that would easily sound like ASD but he is very social and wants to share his experiences with others. He wants to talk WITH everyone about his interests not at them, he draws people in and assumes everyone is as excited about these things as he is.

Last edited by GGG; 09/08/14 09:32 PM.