i do not know much about the early signs but you should get a diagnosis asap. whatever it is early intervention is the best thing.
Good luck.

Since autism is a spectrum disorder, the symptoms vary so much. It would be impossible for anyone, even a Dr capable of making an actual diagnosis, could determine if your son has autism. You need to have an assessment. IMHO, that is best done by a team, usually lead by a developmental / behavioral Pedi and / or a Pedi Neurologist / Psychiatrist.
You can check out the autism society of America, and google some red flag lists for autism, to help you be able to pinpoint your concerns.
Since early intervention can often make a big difference, I would suggest you seek an assessment as soon as possible.
I send you and your son, by very best wishes!!!!

this is similar to what my daughter is doing, I know she can hear me, but she doesn't usually listen, I can't call her and have her actually come to me, she does talk, but doesn't have a conversation with you, and you can't ask her a question and have her answer you etc...but like someone else said, the spectrum for this is so broad it does take alot of good testing to make sure...good luck