Check out this website: firstsigns.org/
There are a lot of resources on the web that list some warning characteristics.
Does he avoid eye contact? have a limited vocabulary? Speak in a high pitched voice? Walk on the tips of his toes? Flap his arms in the air when excited?have a hard time interacting with children? Is he always off on his own?
These are possible indicators of autism. I would call his pediatrician and make an app.
Good Luck