Do talk to his doctor next week and voice all your concerns. I do know this is what happened with my cousin's son who is autistic, BUT your son's behavior can also be normal so talk to your doctor and go from there.
Some things you may consider, is there a reason for him to ask for things like his milk, or do you and the rest of the family offer before he has a chance to ask leaving him no need to? Does everyone talk to him and say the words, "Mama, Daddy, baby" etc., clearly and not in baby talk, or are you communicating with him just in the grunts, squeals and blowing raspberries that he does? He will mimic what he hears. Does he respond when you call him from behind while across the room or do you need to be face-to-face for him to respond to his name? If you are doing these things and notice his responses aren't what they were with your older children maybe it is time for him to have his hearing looked into, even though it appears normal.
Hope all goes well with his check-up and your talk with the doctor and that everything is fine : )