P.,
It is a very common issue for pediatricians to under estimate thier role in getting children the help they need when it is still early enough to do something about it. You want to waste no more time in your child's language development. The tone of your post is like it is because your mommy voice is telling you the right thing, something is wrong, you need help and it feels like the wrong advice to try harder to get him to speak more.
Listen to that voice, and call a speech therapist on Monday morning, make an appointment for a full speech and langague evaluation, and start therapy as soon as posible. When you get of the phone, investigate the EI intervention in California, but don't depend on that to be all he will need, they are only obligated to make him functional, and it will likely be several months before they come to see him and you may end up with the same kinds of advice you got from your pediatrician, because lets face it, that is cheap and easy for them.
The sooner you start intervention the better. The worst thing that could happen is that someone who knows, after testing him to find out, will tell you that everything is fine, and how bad would that be? You will either walk out the door knowing that you are on the road to helping him, or you will walk out the door knowing that he is fine, but you will walk out the door and know for sure that you are doing the right thing instead of rolling the dice and hoping that your child will be one of the few lucky ones who will just start talking one day out of the blue. The child who will do that looks just like the child who needs speech therapy at this age, but you don't want to wait another year without any progress to find out you should have acted now. ACT.
Don't pay a bit of attention to anyone who tells you about thier sister's friend's nephew who did the same thing and is now an honor student. He looked just like a child who is now illiterate and struggling to understand language concepts because he has little reading comprehension. You won't hear about that child on this site often, but I am here to tell you, I work with parents who have that kid every day as they struggle to get them approriate special education services, and it is the saddest thing in the world to go through thier years of evaluation data and see the missed opportunities because parents listened to someone who said "wait and see." Never wait and see when waiting will harm your child if you are wrong. Never wait and see with issues of development. This IS the time to be SAFE rather than sorry, because if you are wrong, the damage is often permanant and life long.
CALL, and I hope that makes you feel better today. Go have a good weekend, talk to him and don't worry about it all being your fault anymore. You can't stop a kid from talking by not being a good Mom, and I know that this has crossed your mind, you would not be worried about him if this was your fault.
M.