You're frustrated ... but is he?! If he's communicating well with you in other ways, he's probably very happy with the situation - he'll speed up when he's good & ready, I bet.
Obvious stuff, so I'm sure you've already tried it, is:
Lots & LOTS of book reading - not nec reading the book, just pointing and naming. Obviously, he'll understand lots before he talks, so asking him to point to x,y,z can help.
Label everything - ie give yourself a running commentary - "I'm just putting mummy's water into mummy's cup" etc (avoid you/me for the minute as they are difficult concepts to grasp)
Encourage pre-verbal communication (eg pointing!)- it basically gives them the idea that it's worth communicating with you - eg. would you like an apple or an orange ? (DS points to orange) Oh, you want the orange, there you are.
Children who are exposed to lots of word-based games/sing nursery rhymes a lot tend to me more verbal
So, in other words, talk lots yourself ... and enjoy the silence before you know it you'll wish they would just SHUT UP again