Hi and welcome!
As you're finding, getting an accurate diagnosis right away can be difficult. It sounds like you're in good hands, though, and you have the right idea about asking WHY a certain diagnosis is given over another. I think you're on the right path. Good for you for realizing right away that you're the best advocate for him.
Please remember that even experts might have a difficult time knowing exactly what's going on until your son grows a bit.
Here's a caution, though: don't get so caught up in this one thing that you don't enjoy every moment. When my son was between 2 and 3 years old, we found he had pretty severe autism. It consumed my every waking moment (and most of my sleeping ones LOL ) for a while. It's hard not to go there ... you feel you have to know everything right now!!
Other patients and parents will post soon, I'm sure.
You are welcome to go and ask some experts: http://birthmark.org/experts.php
Right away, I'd recommend Dr. Nelson, especially if you have questions on the diagnosis.
This fall, November 4, we have a conference that might interest you:
If you can make it to that (it is in California), you'll find a LOT of excellent information there.
All in all, he's a cutie patootie!! Is he a good baby so far? Sleeping through the night yet?