I have an MPH, my mother is a speech pathologist and my uncle is a doctor--and, I'm a mom. So, I have LOADS of backup in saying this: THIS DOES NOT MEAN AUTISM! Autism is actually quiet rare and over diagnosed. Boys tend to develop later than girls in speech, but develop faster in gross motor skills. Listen, he's humming tunes and he's got mama and dada down. He cuddles, kisses, looks you in the eye, right? That's not autism. Might simply be a few things:
1) frenulum (the piece under the tongue) is too short (oral surgeon could easily fix)
2) hearing could be an issue, but if you haven't noticed hearing issues before, I wouldn't suspect this. The speech pathologist should refer you to a audiologist if he/she suspects anything, but I think you would have noticed by now.
3) the part of the brain that strings the sounds together to make actual words isn't connecting...hence, the speech pathologist!
Please don't worry...I have friends with boys (I have a girl) who have had less communication than your son. Six months with a speech pathologist, and all is well. My BFF always laughs and said, "once I got the speech pathologist in and my son was talking, he wouldn't shutup...maybe I should have thought twice". ; )
My mom says that if a parent isn't educated in speech development, they usually don't understand their child is communicating words and phrases to them...example, my daughter was saying "Mmmmmba" at 6 months (and mama and dada). I was on the speakerphone and my mom (being a speech pathologist) heard it and squealed...she wants a bottle!!! Low and behold, I finally understood she was communicating and I'm figuring out what she's saying, I just slowly correct her (I say, "Bottle, you want a Bottle") and her eyes light up. This is a technique my mom taught me. So, I wouldn't bat an eyelash at the autism question...I'd focus on the fact that he sounds very bright, active, and needs a little help forming the words correctly. Congrats, you might have a little musical genius on your hands.