Hi,
My dd2 was similar to your son. My other children all spoke early, but the latest trend has been baby signs and I decided to experiment with my dd2. She did well at first, and had 25 signs by her first birthday. By her second birthday, she still had 25 signs and about 20 words. I was very worried, but her pediatrician said she will be fine, etc. The months passed with absolutely no improvement. Suddenly, at 28 months she decided to talk. She is now 30 months and talks all day! She speaks in full sentences (e.g., "I don't want any mowe mik, I'm full, I want mik tomowow" she knows the alphabet upper and lower, numbers, colors, shapes, knows and can count up to 20 items, and has started to read some words. She went has changed so suddenly that its incredible she is the same child. I read Einstein Syndrome when she was 25 months, and it seemed incredible how those children progressed, but now I am witnessing the phenomenon. Keep talking and reading to him, he's learning, and before you know it, he will surprise you as well!