My son did not speak until he was two because he had chronic ear infections that made it impossible to hear properly. The fluid on his ears distorted his hearing to something similar to sound waves underwater. As a result, he even developed his own language based on what he was hearing, which we eventually spoke with him, and it sounded like words but garbled. Now I understand that it was the "underwater effect" of the fluid on his ears from the ear infections.
He finally grew out of it at around two years old. It had something to do with his tubes. Because he was our first and we were young parents, we had no idea that his speech development was not on par with all of his other developmental milestones which were at or before their expected date.
We learned of this after taking him to the Michigan School for the Deaf at the suggestion of a family member who explained that he should be saying much more by this time given his intelligence and they would even sneak up behind him and make loud noises, to which he would not respond. This worked sometimes and sometimes he did hear the sounds, but his lack of response correlated to the times his ear infections were the worst, which we finally figured out later.
This may be something you want to check into. Again, if the cause may be unnoticed ear infections, as was the case with us, he will be able to hear sometimes, so you may put it off, like we did, but the result of having his developmental years without hearing resulted in our son's using other ways to communicate, and now, although he is hearing fine, he tends to not rely on that sense as one normally would. This obviously inhibits his verbal retention when given commands, so we must interract with him in more kinesthetic ways.
Still, he is 15 now, and other that being a bit inattentive to verbal cues, he is a wonderful young man and is quite a bit more sensitive, mature, and compassionate than most other boys his age since, even after regaining his hearing, he was treated poorly by many adults, who, regarding his age and size, expected much more from him in the area of following directions, not realizing that even though he was 3 or 4 years of age, he had only been hearing for about a year, so their verbal command expectations of him should be what they would expect a one year old to obey. When he didn't understand their directions they would call him rebellious or defiant when he really just didn't know what they were saying to him.
I am not sure if our experience sheds any light on what your son may be going through, but perhaps it will be a stepping stone in the process of finding out. I wish you the best, and feel free to contact me if you have any questions, although I think I gave you as many details as I have. On a side note, however, I have an uncle who didn't say a word until he was four for no reason at all and he is a brilliant man today, so you never know... I hope that helps! :)