S.H.
My son was like that and he is fine.
My son was not on the low side for weight gain though... but he was a "grazer." Meaning he did not eat large quantities per each meal... instead (like my Husband), he eats a little throughout the day. And he only eats until full. Which is the ideal manner in which a child or anyone, should eat. So that they know their body's cues... for hunger/fullness.
My son was also a picky eater. He is 5 now, and is less picky. He eats more variety. Naturally.
Then, there is also this article which you may read for added information:
http://abcnews.go.com/Nightline/extremely-picky-eaters-st...