Lots of face to face time, eye contact, telling stories, nursery rhymes or singing, or just chatting about what ever. Don't be afraid to exaggerate your voice or facial expression, or just be a funny mummy. Even things like clapping hands, making shapes with your tongue and oohing and ahhing and smiling when he copies you all help with learning that talking is something people do together, kwim?
ditto, my child didn't really speak until three, and it worried me at the time - although her comprehension, humour and communication in every other way was great (she would act out what she wanted, people KNEW what she wanted, even without words). Everything Marydean says, i second.

it was so weird, after three years of just "mum mum", "dad dad" and "mummy booby" (for milk), she turned three, and it was like a waterfall was turned on! all of sudden, complete sentences, from nowhere.

i remember her 18 month health check, when SUPPOSEDLY, babies are meant to have ten words by then (and my child sure did not). They all learn at their own pace.