Dear C.,
I am sorry you are dealing with such a stressful time. I am sure you are just wracked with worry over your son!
I think a lot of people here are jumping the gun telling you your son has Autism. I agree with Katrin in that not speaking is only ONE symptom of the disorder. If your son is still able to relate socially to others around you, is able to look you in the eye, be touched, doesn't do what they call "stereotypic" movements (rhythmic movements made repeatedly that have no purpose to them, e.g., rocking back and forth, flapping hands, walking in circles, and so forth), then it isn't likely he has autism. Of course, several things should be ruled out with that being one, but don't let others convince you to jump the gun and "arm-chair" diagnosing your son!
Another person mentioned getting his hearing checked. I agree. However, you did mention that your Dr. did not find any physical reason to your son's symptom of not talking and that the Dr. believed the cause to be emotional. From all that you've described and the timing of your son's not talking, the divorce and subsequent inconsistent visits by his father do sound to be the cause of this symptom.
I would strongly suggest, after you've ruled out all physical causes, that you seek out a good Play therapist. Play is the medium by which a child as young as your son uses to communicate and a therapist well trained in this medium will help you to unravel how your son feels about all that's happened to him that he has perceived to be traumatic (and no, I am not assuming abuse as some other poster also stated). Play therapy will help him to vent and express his feelings that he cannot speak about, and thus help him come to a resolution.
If this is indeed Selective (not elective) Mutism, then play therapy should be able to resolve this quite effectively. This diagnosis only fits, however, if he only does not talk in certain environments, but still does so in another in which he feels safer.
Good places to look for a well qualified therapist (make sure to type in the search field "play therapist") would be to search the following therapist on-line directories:
www.therapistfinder.com
www.psychologytoday.com
and the Association of Play Therpy.
Sincerely,
S. M. Wolf, M.A.
Licensed Marriage & Family Therapist
Anahiem, CA