My son is also a poor eater since birth. He's 3kg at birth and now is almost 9 months but weigh not even 8kg. He's always been on the bottom percentile. He was mixed fed since birth, 50% breast and 50% formula. However, after the 1st cocktail vaccination, he went on nursing strike, totally refused bottles and pacifier. I was worried sick and was on the edge of depression seeing him loosing weight everyday.

Now he's eating solid (plus breast feed)but still very very picky and throws up easily, at least 2 to 3 times a week, especially if he doesn't like the food and when he cries ( I guess he knows I'm very concern of his food intake and this is the right button to press). I've tried the CIO method to let him learn to fall asleep by himself but ended up a little human fountain with puke all over his face, pillow, grobag and bedsheet. I have consulted a few pds and lactation consultant but they all say he appears to be healthy (he's not been sick since birth) and active and the height is meeting the average 50%, there's no point to worry. Every baby is different. Of course I can't compare mine to those little chubby angles. He's a happy boy and loves to laugh and play, so I tried to enjoy motherhood the maximum(especially when they're still tiny and cling to you), play and have fun with him instead of worrying how many grams he puts on every week or months.

My MIL was a poor eater as well, she was only 10kg when she's 2 years old. She told me she hated to eat, she'd spit out the food once the maid turned her head away. But now she's a big lady with very very good apetite. So try to rule out all the possible physical problems and enjoy motherhood, these little things grow up really fast.

Wish you all the best.