J.,
DOn't worry too much about your son, it's common for this to happen with twins!!! I have g/g twins that will be 4 in November, and we had a similar issue. Even to this day, one is much more vocal that the other. Sometimes one is the more "dominant twin", aside from communicating, is your son the more easy going of the two? Another thing we found with the girls, is that the one who talked more actually talked FOR her sister. I laugh when I ask the "less vocal" one to count or recite the alphabet she answers me with "I don't know" and her sister tries to take over, now I have to take a little more one on one time with her and distract or have Daddy or Grammy distract the more vocal girl to get the less vocal to trust that she knows just as well as her sister what she's talking about (because we KNOW she does as when she's alone in her room with the stuffed animals, or playing quietly on the floor with her dollhouse, she is constantly teaching them to count, the alphabet, colors, shapes, anything she can think of.)I honestly think we didn't hear alot out of her until she was at least 2, maybe even 2 1/2, we went through the hearing, vision, developmental, any test they could come up with..... and it turned out she was listening, and she wasn't going to say anything until she knew she could say it correctly, and her sister wasn't going to speak for her.
SOme lil ones are just late bloomers, I'm sure if there's reason to be concerned, your Dr will let you know. If you're still very concerned, you may be able to call your state Dep of CHildren or Social services and have him screened for whatever your state's equivilant to Birth to three (in CT) is.
Good Luck and enjoy those babies!!!