Everyone is different but always remember that there is light at the end of the tunnel of raising a profoundly autistic child.
My nephew is 32 years old, diagnosed with autism about age 2, and cannot "volunteer" words. I don't what else to call it. He knows words and what they mean but he cannot just pull them out of his brain, string them together in a sentence, or voluntarily use them to express himself. He can ace true/false and multiple-choice written questions, but he can't do fill-in-the-blank tests without a list of potential words to fill in the blanks. He cannot write an essay as is required to get his GED.
He expressed himself with echolalia when he was young, and he still does that to some extent now. He has always been able to perfectly mimic the voices and recite verbatim the dialogue of Sesame Street characters and the Muppets. When he was young, I realized that he would recite dialogues from the Muppets to answer questions and to express emotions that he was feeling. I think he is so smart that he figured out to communicate that way because he cannot just voluntarily speak words or sentences. He sometimes sings hymns at church in the voices of various Muppets. He is confused by pronouns, not understanding when to use her, him, she, he, they, mine, our, yours, you, etc. For example, I recently asked him "when is your birthday?" He looked confused and pointed at me, like asking me if I was asking him when is MY birthday, and I had to point at him as say, no, when is YOUR birthday, not MY birthday. He then understood and told me the day, day of the week, the year and the exact time of day he was born. I suppose he's seen his birth certificate at some point.
I think he has some disconnection in his brain when it comes to processing language. But despite those problems, he is a most wonderful person and companion, and he knows that I don't care if he speaks or not. We have always been able to communicate sufficiently to do most things although our type of communication is very different from other people's. I love him dearly just as he is.
With patience and loving support, your son will find his voice, whatever it may be, on