My older brother used to do this also, and for YEARS... until he was in his teens. There's nothing wrong with him, it was a comfort thing for him. I really wouldn't worry about it, unless you're seeing he lacking developmentally.
Rocking is not a big deal...I've read that about 20% of toddlers rock in some form or another, and it's just a repetitive comfort thing for them. They usually stop doing it by age 4 or 5. If he does it a lot while watching TV, he's probably just 'zoned out' and not even realizing what he's doing.
As for autism, you should only be concerned about this if he is having serious developmental delays or doesn't interact, etc.
Sometimes very minor cases of Autism do the rocking motions.
Functional Autism is not bad the child would just have to foster different behaviors. Truth be told many functional people with autism are usually genius in one field or another.
One of my nephews used to turn his head from side to side when he was tired. This started under the age of 1 and continued until he was at least 6 years old. He is now over 30, smart and healthy. I wouldn't worry about this, especially if this is the only behavior he is displaying. This is probaly comforting to him in someway. Some kids just enjoy movement.
well you can't just say something so PROFOUND like autism and give no explanation. what is it that makes you say that?
Probably nothing, could be autism.