The first question that comes to mind for me is ...is this the percentile he's always been at or has he really gone down in recent well-child visits? If it's the percentile he's always been at, I think there's much less reason to worry. Is his height along the same lines as well?
Some things you can try checking -- how is his..um..elimination? :) If he's constipated a lot or simply just holding it, there's only so much tummy space! If his height is worrisome, too, you could always have his growth hormone checked..but a GI doc might be a better first approach (go for the simpler stuff first:)).
How many ways have you tried to present various veggies? I grew up eating mostly canned (the only fresh we had I would eat were tomatoes and green peppers) and honestly hated most vegetables myself.
My step-son was stauchly against vegetables of any kind until I tried boiling fresh veggies for him. Now he devours them--and not just boiled. Fresh green peppers make great finger food--as do *surprisingly* squash and zucchini! If he'll eat cheese, you could even try a "mini pizza" where you put a slice of zucchini or squash on a Ritz or saltine and put either a piece of an American single or some shredded cheese on top -- microwave long enough to melt the cheese..and viola..kid friendly but relatively nutritious :)
Toddlers (sorry, your baby is now a toddler :)) are notorious for picky and light eating, so try not to worry too much. :) As long as your son is staying on the same basic percentile then just enjoy this time--he'll eat you out of house and home soon enough! :)