I'm pregnant with my 7th child right now. I have a bit of experience under my belt (grin).
Some of mine didn't "babble". I had one who, literally, went from the VERY occasional "mmmm mmm mmm baaa" to FULL sentences. There was NOTHING wrong with my child! Just liked to figure it out before showing off to the family (STILL does this. Won't see this one try anything or practice, and then one day will do it like a pro!).
Keep doing what you're doing. Talk to him. Read to him. When he jabbers to you, repeat the noises he makes back to him. If you respect his current "language" (jabbering), he'll learn to use yours, as well. Trust me. It will be fine.
PM me if you'd like to talk or think you're at your wits end. And ignore that pediatrician. They tend to go by "guidelines" and forget that all children fall within a bell curve!