Hello, talk about absolutely everything! If you are doing your dishes/washing etc (as well as usual chit chat) talk your son through what you are doing. If you are out and about point out everything you see and chat about it, If you are having a bit of telly time talk about everything you are watching, Get the books out read the stories and talk about the pictures. By saying all of this I'm not implying that you don't do this already! My Dd's vocab wasn't too great (and no she wasn't stuck infront'o'telly all the time she just had re-occuring ear infections since she was a baby ) but has improved greatly since I started talking her too death about absolutely anything with her.
What Crazee said. When you watch a film / TV prog, talk about what's going on. When you read a book, connect it to their lives... "Do you remember when..." or ask them what they think might happen next, or ask them why that happened, or why she felt like that, or how they knew that. When they talk, include an extra word in your reply.... so dc "Can we go to the shop" Adult " Yes, we are going to the supermarket"... extends the vocabulary or dc "Look, a dog" Adult - "The brown dog ? I think it's a spaniel" etc.,etc.etc. Also read and read and read to them - it extends the language they hear, and also often raises questions about things mentioned in the stories.
Thank you all for your valuable replies. And yes, I too have been reading stories to them and making them understand in it in complete sentences. Now I would be doing it with more dedication. There is no reason behind me being worried, I just thought other moms were doing something extra.