We went through the same thing with our son. He exhibited similar habits that you describe. We were worried when he did not talk, not even to say mama at 2 1/2 years old. He was diagnosed with Apraxia after undergoing 3-4 months of speech therapy. He was admitted to the help me grow pre-school program through our local school district and showed no progress. We dealt with horrible temper tantrums. For this, I recommend sign language. There is a great set of DVDs called signing time. They were originally created for kids with hearing disorders, but they are fun and all three of our kids loved them. It was a tremendous help with controlling the temper, because we gave him an outlet to communicate. We also used site cards, letter & numbers. We provided daily games to encourage his vocabulary and worked with him on writing basic letters.
On the medical side, I am not sure if they have performed an EEG on your son. They did this to Adam. It was horrible, I would not recommend it, only as a last resort. I could not stand to see a child put through what you would think is such a simple & painless procedure.
One day for whatever reason, out of the blue started speaking (during the summer), when he went back to school the teachers were shocked. Don't get me wrong, he is still very OCD, perhaps ADD and has just about every speech impediment that a kid can have but he has come a long way. We now talks, writes and believe it or not can read. Somehow is was taking it all in and putting it to memory. He will be 4 in march.
I will tell you that there was history of late talkers on with my husband's brother, who did not speak until he was 4. I guess they did not make a big deal about it in those days.
I am happy to answer any other questions that you may have.