On the mend!

Well....we are on the road to recovery! Jamie and Ryan are both on antibiotics and getting better! I am still hoping for an uninterrupted night of sleep though!
Seriously, Ryan hasn't eaten well since he got sick (over a week ago!) and it has been a huge struggle getting him to eat, even now that he is feeling better. He has seriously lost weight and you can tell in his face and when you pick him up. So, when he wakes up at 3 and wants crackers, what do you do???? He is seriously hungry because he only ate 4 bites of dinner (after an hour and a half struggle to get those 4 bites down!) We gave him the crackers, but we don't want to create bad habits either.
We are offering food whenever he wants it, but he won't eat it! Tonight was better. He maybe ate 2 or 3 ravioli, 2/3 of a nutrigrain bar and half a sippy cup of Carnation Instant Breakfast before bed. I'm hoping the extra calories of the instant breakfast will get him through the night.
It's especially hard, since Ryan has always been on the low end of the weight scale, even though he's been a good eater. We are trying to get the ice cream, cheese, peanut butter, etc. into him, but what can you do if he won't eat it??? Pizza is his FAVORITE food and he took 4 bites last night--even with the promise of ice cream.
Any tips on how to get a child who won't eat to gain weight?