Hi,
My daughter had the same experience and I only regret that I didn't take her to speech therapy much earlier. Like you, I did 'all the right things' for speech development. I even had the additional benefit of having enrolled her in an early childhood development study that helped me stay abreast of all the latest techniques. Doctors and educators all agreed: wait it out. But a simple trip to therapy was all we needed, and it's so painless.
The doctors finally gave up and sent us to therapy when she turned 3. It turned out she had a 7 year-old's vocabulary crammed inside her little head and she didn't have the motor skills to pronounce what she wanted to say. It all made sense, once the therapist tested her. She likened it to a log-jam between my daughter's mind and her mouth.
Perhaps this is like your son? My daughter began speaking at 10 months with single words. She then progressed to two- word phrases. Next, she SHOULD have progressed to three-word phrases, but she didn't. She skipped straight to LOOOONG sentences, complete with excited gesticulation and inflection. The problem was that only three or four words out of 20 were intelligible. (And that was a good day!)
The look on her face convinced me that this kid knew what she was TRYING to express. The moment our faces registered a blank look, her whole demeanor changed. She knew we didn't get it... again. It was like watching a balloon deflate. It broke my heart. That's when I saw it was changing who she was and the way she saw herself. She started getting quieter, shutting up sooner, she stopped trying to be heard. She was giving up.
I went to the doctor again and didn't ask him a thing. I told him what we were doing. Period. She was in therapy a week later. It set her free.
The therapist determined in one visit that her vocabulary was huge, but her (speech) motor skills were weak. She skipped the stages of development she needed to train her muscles to enunciate -- she had too much to say and no way to say it. Can you just imagine having so much trapped inside you, wanting to be expressed? How lonely she must have felt in our crowded house.
It was such an easy thing to fix -- seriously, within two months she spoke clearly enough to be understood. Within six months she sounded pretty much like any other three-year-old, she just talked about a larger variety of things. And she still talks a LOT.
I just can't believe I waited until she was three -- I should have demanded speech therapy the minute my gut instinct said she needed it. I could have saved my daughter thousands of deflating, frustrating, isolating experiences.
Whatever the problem, a therapist will be able to assess it's source and treatment -- or at the very least, direct you to a specialist who can.
I speak humbly from experience. Don't wait it out. Go with your instincts. Don't ask for a referral, very sweetly demand it. My daughter still looks back on her short time in speech therapy as the time in which she was finally heard. And all it took was someone who knew what to listen for.
Best of luck! :-)
PS: For whatever reason, my daughter found Spanish easier to enunciate than English, even though we speak English in the house. During those gobbeldy-gook years, she spoke Spanish more frequently -- and more clearly -- than English. Especially in a crisis! Just fyi; I have no idea how it could help, but you never know. :)