Oh good Lord!!! Give the kid a break! He's only 6.5 months old! HA....I agree...doctors work with formualas and guidelines that not all kids fall into. ALL kids are different. Even within families, they are all different.
My first born is a boy, he's now 7. He spoke the usual few words boys do growing up...ball, nana (for banana)...some other simple words ...but not necessarily very verbal (which I'm now learning is a boy thing). Yet by the time he was 14 mos. he was very distinct in his pronunciations of every word and by 16 months already had short sentences down. It wasn't baby talk...he was just very clear. He could point to anything in a book and tell you what it was.
Now my daughter, who is now 18 months...has been babbling and carrying on conversations (just like a woman, ;o) ) since she was 4 months old!! When she first started up we freaked!! ....thinking how can this be that SO much verbage is coming out of this little infant!! But part of it IS the difference between boys and girls. And yet now at 18 mos. even for as much as she talks ALL day long....you can't understand anything she is saying....hahaha. She has her small library of words that WE know what she means....but in comparison to her brother at that age - worlds apart.
But yet, her specialty at this point is her fine motor skills, her very stable walking, climbing, etc. abilities, carrying very heavy items without stumbling. All of 'that' ability builds critical thinking abilities.
So, relax if your son isn't following some guideline based on what the doctor said.
It's very easy to keep testing him to make sure he isn't having any hearing problems. When you are behind him talk to him low key....does he turn around? Does he jump when there are loud noises? Keep watching him...you'll know.
But just give him more time to find his own way.
My daughter can follow every instruction I give her...so I know she understands language and connects everything. And just within the last 2 weeks I have heard her improve more clearly on certain words. So she's getting there, and as she approaches 2 yrs I'm sure she'll be even more distinct. But I learned from all the Moms around me that their boys were also not that verbal in their first year. And the girls although babbling forever it seems ...still weren't as distinct as the boys were after the first year.
But again, not a cut a dry way either way. I will say though that it is really important to read to them already. Alot. I started reading to my son at 4 weeks. Most of the time I'd just describe the page with it's colors, characters, etc. They learn from your voice inflection. It all goes in and eventually it all processes and they get it. But you need to be hands on and don't wait for teachers or preschool for them to learn.
My son learned his A,B,C's, 1,2,3's, colors, and shapes by 2.5yrs old. He's in first grade but from day one they've put him in 2nd grade advanced reading and math. And that didn't happen because of the school system. I consider my kids 'home schooled'. Going to the building every morning is just a formality.
My daughter is learning all that stuff now....and they DO get it. So if nothing else read to your son ALOT right now. Believe me it will not be in vain and from that he will hear more sounds and begin to mimic more. You should start hearing more out of him...and if that happens then you know he's getting it.
Good luck...I'm sure you'll see it'll be fine.