How is your son with computers?
If he's pretty good with them, I believe he would benefit by having what I would call a "talking machine," but what most people call an "Augmentative and Alternative Communication Device" designed for kids his age.
Basically, at his age, there will be symbols for common requests and common everyday things in life. All he has to do is push the button, and the machine will do the "talking." This would also be a good way to teach him to type. He can type in "cat," say, and machine will say "cat." A symbol of a cat will probably come on as well, accompanied by a "meow."
In most of them, probably, an individual "voice" can be created.
Basically, he can "talk" using this machine. Especially if he feels a desire to communicate, this would lessen his frustration.
The only trouble: some of these machines are quite expensive--but, perhaps, the school district will pay for it.
I don't really like PECS all that much--unless you know he will have trouble speaking later in life.