Is my 5 yr old Autistic/Aspergers/HFA?

My son to me is a normal 5 yr old happy, smiling and loving boy. The only problem I notice in him is his language is not on par with other 5 yr old kids. When it comes to a social situation, he is not very good in making friends but is not very anxious or nervous when he sees a lot of kids or a big crowd. But when a kid comes and approaches him continously or say if he/she visits our home he is totally fine with the kid playing, sharing, taking turns and talking. He does take some time to play with same age kids, but he can play with only one kid and 3 is a crowd for him.
He is very comfortable with kids in "HIS" environment.
He knows that he is being called but still doesn't respond. He does say that he heard me calling him many times.responds when i change the tone
He knows when he is making a mistake and apologizes when asked to do so. he has good fine and gross motor skills, not very aggressive, sensitive to parents' feelings.He is fascinated about trains,cars,airplane,firetrucks and clocks. He has just started asking my "why and how" questionsand my help at times
He talks in sentences nowadays with 7-8 words and is able to follow instructions/directions.
We speak a different language at home. My son goes to school for only 3 half days. He started to go at the age of 3.5. We didn't have much social interactions or playdates when he was young and even now. So I feel he finds it a little bit difficult in playing with kids. He understands sharing and taking turns but with great difficulty.He takes some time to play with the other kid but later he does play together and have lot of fun with kids after some time.
I have started teaching him at home and he knows atleast 20 letters and numbers till 10, traces them well and has even started learning words. This has happened only in the past 2 weeks. He is able to sit and listen to me continously for 1 hr! .At school he is often found alone and aloof and standing as if he doesn't know how to approach other kids.

[Jason C Bourret, Ph.D., BCBA-D]

From your description, it’s hard to tell if an autism-spectrum disorder diagnosis would be appropriate. Many of the characteristics you describe are seen in typically developing children as well. However, if you have concerns, or you believe your son could benefit from more intensive teaching, I recommend having him assessed. Your pediatrician should be able to refer you for an assessment, once you express your concerns.