Our son just turned 2 and he doesn't talk very much either. We just had his 2yr check up and the Dr said that as long as he can communicate with us somehow and we can tell that he understands what we are saying, and can take simple directions, etc., that is what is important. She said that kids vary so widely in speech development between the ages of 18 months and three, but most of them are about even by the time they reach 3. She also said the thing you should watch out for is that they are always adding words, even if it is slowly, and not losing them. We've noticed our son has added several more single words to his vocabulary in the last month and has very recently started stringing a couple words together, like "more trucks". Our daughter was speaking in sentences by the time he was her age, so I try not to compare. Kids are all different. My mother-in-law says my husband was a late talker, and I've seen enough family members who were late talkers who eventually caught up and were very bright little kids, so I don't worry about it. Our boy was a late walker too... almost 15 mos. Now I can hardly remember him not walking... and now running... and climbing... and I'm sure it will be the same with talking too.
One suggestion that may help you and your son's communication frustrations in the interim (if you haven't already tried it) is to incorporate some simple sign language into your routine. We use(d) some like "more", "stop", "milk", "thank you" "change diaper"... I picked up a video from the library when my daughter was little called "signing with your baby" which teaches a lot of the signs and the methods for teaching them. As they learn to say the words for the signs, the signs sort of drop off.