I can totally understand where you're coming from! My 15 month old was in the 75th percentile for the first 6 months, then began to gradually decline. Last month, she was at the 15th percentile. I feel it's only a matter of time before she falls off the charts...She eats very little, and has no interest in eating at all, although she loves snacks. At one point, she even refused juice! I mean, what kid refuses juice?! Anyway, I spoke to the pediatrician, who suggested Pediasure after meals. My daughter doesn't like Pediasure, but who knows, maybe your son will. Have you tried it?
I have a friend, who happens to be a pediatrician, and she has a three year old boy who eats very little. They came over for dinner and I served lasagna, and he ate less than my 15 month old; yes, the one I mentioned above. He's not even on the charts. I bought him an 18 month old size polo shirt for his 1st birthday, and he's able to fit in it now. All the tests were normal so she basically says all she can do is continue to offer him food and have him eat as much as he's willing to eat. My friend doesn't seem too concerned since she and her husband are on the smaller side. She's about 5'2" and her husband's probably 5'7". Both are about average in weight.
I'm beyond frustrated about my daughter's poor eating so I spoke to some people who told me the stuff they tried that worked:
I spoke to my neighbor, who told me that when her daughter was around 3, she ate very little. Her pediatrician suggested that once her daughter started playing with her food/showed little interest in food, take the food away and don't give her food, except a light snack until her next meal. My neighbor said she followed the pediatrician's advice. After 3 days (3 meals per day), her daughter started eating everything given to her. Is this something you're willing to try?
I also have a friend who has a 2.5 year old boy who eats poorly. She said that when she lets her son watch TV while eating, he eats better. I also tried this with my daughter and it does work. Since I don't feel comfortable letting my daughter watch television with every meal, I try to distract her/entertain her myself as I feed her, and this also works.
I have another friend with a 2 year old daughter who is picky. She said that for her daughter, it was a texture issue. So my friend blended all the foods for a while, which she said worked for her.
Good luck!