I agree with your ped, refer at 18 months if no progress. Some kids are late talkers, some have language delays, but by the time you find out by waiting, you have wasted months, if not years, of time they could have been getting therapy. And when it comes to language therapy, the earlier, the better. A SLP will be able to look at the whole picture and help determine which is the case. Most 2 year olds should have about 59 words, and be starting to put two words together.
I would do 2 things:
1) Talk about everything you do. Talk til you feel ridiculous. Talk and talk and talk some more. Comment on everything you see. It really helps.
2) See if he will learn some signs to communicate basic wants and needs, like eat, drink, more, mommy, ect.