DI have worked with many DS children Nd I have learned that in time and with proper speech therapy they do learn a vocabulary tho not all Re the same. I have a special needs daughter and know of very few with the same that actually speaks just give it time. He may surprise you . Try repeating things to him repetition is amazing !! Keep us posted and let us know if he starts to talk good luck!!
My son is 4 1/2 years old and actually started talking about 9 months ago. It is not clear yet so most people don't understand him but we also use Sign Language with him (Signing Times DVDs) and it helps quite a bit because he isn't as frustrated. Jamie314 is very right about repeating things a lot.
Also if you feel he isn't understanding how to make a certain sound, you can put his hand on your throat as you make the sound.
Children with Down Syndrome do things in their own time so you have to be patient. My son didn't even actually walk until about 3 days before his 4th birthday but now he's a pro!
My 3 yr old started speech therapy as soon as he turned 2. It was great teaching techniques for us parents and older sister too. So when we play at home, we repeat and put into practice what we learned. Repeating over and over, is a must and eventually he will get it. Don't give in to what he wants until you say the name 3-4 times minimum and you see him making an effort to repeat it or sign it to you if you do.signing. Sign language dvds are awesome. My son doesn't do much signing, he'd rather say the word. Lol. Good luck, hope these tips help.