I have read this topic from time to time as it's gotten longer and I just thought I'd put in my 2 cents. I'll use my niece as an example - she's 8 years old. In terms of listening, she almost NEVER listens...which can get on our nerves quite a bit. I agree with Brick on this one too, kids sometimes just don't listen, my niece being a perfect example. She doesn't have autism, or any other mental disability, she just doesn't listen and she knows it irks us to no end.
Also, as someone here has mentioned, mental abnormalities can be picked up in the womb before the baby is born - was that mentioned at all when you were pregnant? I honestly think your son is just a normal child growing up. Maybe he just needs a little more time to develop that's all. I'm sure he will be just fine, just keep us posted about how he's doing.