Read and talk to him, constantly, as much as you can, about anything and everything.
And in the meantime, start looking at speech therapy. It's painless, and can really help a lot. Look up early intervention in your state, and see what kind of preschool-age programs they have. There's usually one done through the school system; they'll do a complete evaluation of speech, development, vision, and hearing (they'll pass on hearing if he's had a recent hearing test done), and based on those results, they'll do a more comprehensive evaluation for any area in which he seems to be lagging, and then start therapy (usually at whichever school or child development center participates in the program and is most conveniently located for you). You can also look into Scottish Rite's RiteCare and see if there's a center near you and what they might charge (it's like Shriner's, but they do speech/language pathology, may be need-based).
at 2:43 PM on May. 11, 2011