Here are three great articles for you to read. None of them are very long, and each of them are worth reading.
Some things I think the articles talk about are having your toddler help you in the kitchen and choosing meals. He can wash vegetables, pour ingredients, shake spices, etc. He can hold onto the fruits and veggies in the grocery store cart. Basically, he'll be more interested in trying new things if he is more familiar with them and if he helped prepare them. Also, you can give him a choice of two things: "Do you want corn or sweet potatoes with dinner tonight?" You can also offer either peas or carrots that you know he will eat along with whatever side he picks. If he chose it, he's more likely to eat it.
However, as some of the other moms said, at least he is eating peas and carrots. That's great so far. Remember, it can take up to 12 times of introducing something before some kids will try it. I know you said he used to eat these things before, but it may be more of a picky toddler thing now, and he is probably trying to show some of his own authority in the matter, which is perfectly normal. That may be why letting him choose between two other items may be helpful. Also, try not to get into power struggles with him. That could cause him to resist even more. Just keep offering, eat different kids of veggies yourself, and don't make a big deal in front of him about him eating his veggies. As you and the rest of the family are eating your veggies, you can casually mention things like how green beans, etc. help you grow big and strong.
Here are the articles:
http://www.askdrsears.com/html/3/T030800.asp "Feeding Toddlers: 17 Tips for Pleasing the Picky Eater" It has good ideas for introducing new, healthy foods to toddlers.
and
http://www.askdrsears.com/html/4/T040200.asp "ABC's of Teaching Nutrition to Kids" I thought this one was especially good because it teaches parents how to make learning about healthy food fun for kids. It talks about how to educate kids so that they make healthy choices on their own later on and so they know which kinds of food are healthy and why. The only thing I didn't agree with was that he suggested rewards, but everything else was pretty good advice. The article talks about referring to "grow" foods for kids. Your son is not too young for this. The earlier you start, the better.
http://www.nytimes.com/2008/09/15/health/healthspecial2/1...
Best wishes, and great job for making sure your kids get their vegetables!