Nearly 3 year old not talking - should I be worried?

I have not had the same experience, but I just wanted to say that I think you are going about dealing with it the right way, with the testing and therapy. I would definatly continue that, and if one doctor doesnt give you an answer you are satisfied with , then find another and another until you feel your son is getting all the help he needs. I will keep you, your son and family in my prayers. I hope that you find out something soon. I'm sure just know what's wrong will be quite a relief. Good Luck.