My 8 year old was recently diagnosed with Aspergers (and bipolar disorder). We started noticing odd social behavior around age 3, but had to wait 5 years before anyone could figure it out. Everyone said she was just immature. She reads A LOT, and as much as she wants to have friends, its awkward for her. She sees things as one sided and has had a lot of issues in school, but having a diagnosis has helped us adapt to her. Kids at school are still put off by her, but hopefully as she gets older she'll be able to control some of her issues. I'd def have your son checked out, but at such a young age it will be hard to pinpoint his issues. Good luck!
My half brother has Aspergers...when he was 4 he had a lot of the same things that you are describing...He is now 20 grad 2nd in his class and is in pre med...VERY smart kid but he went through that when he was little where you would be out and he would yell because he wanted something dif they what got ordered for him...Once he had a tantrum because there were no meatballs in the spaghetti...it is like he didn't care if he caused a scene.. had NO friends...he had fixations on things like the movie Mrs Doubtfire...drinking tea...weird statues...The Three Stooges...My dad had him later in life and he and his wife did not seek help for a long time....until he was a teen I think and by that time no one wanted to be around him because he had no impulse control and he would ask prying questions and sexual innuendo things.. or want to argue politics!!!.He got on the right meds and he was fine...I think it is very hard to diagnose....Your son may be just immature but I would not wait knowing what we went through..I would at least ask his pediatrician what he thinks....ON the other hand, I have seen kids do what you have posted and don't have it... Our grandson will be 5 in Nov and he would NEVER sit and read...that is a good thing if your son does that...he is a whole other story... Good luck...