I'm glad to hear your youngest is growing :)
As for the older one, I'm sorry to hear he's not eating. I was the same when I was younger, I would hardly eat and never gained weight. So my parents stuffed me full of the only foods I would eat; Chocolate milk and chocolate pudding :) Obviously it's not the healthiest diet, but it was rich in calcium, I got the dairy I needed, and even the calories. It wasn't a miracle cure, but as long as my parents could get me to eat something that was normally a treat for me, they were happy. Does he have any type of food that he would want to eat whether he was hungry or not? Sometimes what the food is is not the only thing that matters, and fatty foods can be healthy for malnourished children (as I was).