I have to agree with the other three ladies. The best things you can do for him are teaching him signs, tell him everything you are doing as you do it. Ask him questions and give him an answer, and read, read, read.
The more exposure he gets to language and vocabulary, the more he will be able to learn. Also, repetition is the key for small children. The more something is repeated, the faster a child will learn.
You don't have to go out and buy a whole lot of books or tapes on signing. There are internet resources that are free, books on baby signs for under $20, and some school systems or libraries give free or low cost classes on sign language.
Also, when you say a word and it sounds like he's trying to say it, say the word you think he said back to him. That will let him know you're trying to understand him and give him a good model of pronunciation.
As a final step, you can have him evaluated for free through the local public schools. Contact the special education department and ask about an early intervention evaluation. That way, if they think there is an issue, he can get therapy services for free.
Hope this helps.
A.