Hi M.,
Well, here's the bottom line. If he has energy, is acting normal, is hungry and thirsty, playful, asking for foods, he's fine to eat nearly whatever he wants. Yes, that sounds like I'm crazy and completely contradicts what everybody else is telling you here. Here's the deal. You said he eats all day long with no trouble and then throws up at the end of the day and "gets rid of all of it". Well, that's just not possible. His body is digesting what he's getting all the time. So whatever he throws up in the evening is whatever he has eaten in the last couple of hours only. He may still be losing weight, but most likely that is because you have changed his diet and he's not getting the normal foods and the amount of foods he normally gets. So he's not wasting away. Not even close. At 3 1/2 years old, he can tolerate a 6 lb weight loss with no problem. However, and here's the part everybody is going to disagree with but it's the truth, you need to start feeding him completely normally. The longer you go without feeding him normally, the harder it is on his belly when you do give him regular foods. Getting through the flu is not rocket science. If he throws up in the evening once or twice, oh well. Give the poor kid what he wants to eat, make sure he gets plenty to drink, and when/if he throws up, clean it up and move on. The key is that he is acting completely normally otherwise. So it's time to get back to normal. The stomach flu is a virus and a virus can last a day, a couple of days or linger for a month or more. You can't continue to fret over his foods for that long. Again, the key is that the vomiting is the only symptom. Here's what to look for. #1 is pain. If he starts complaining of severe, constant abdominal pain, that's a problem. Also, fever of 101 or greater, lethargy, constant vomiting (that means throwing up whatever he eats within 15-30 minutes of eating it, so vomiting only in the evening is not constant vomiting), sunken eyes, no tears when crying, no urine for 24 hours, no pooping for more than 5 days, refusing to eat or drink anything at all, blood in his vomit or blood in his urine or stool. If none of those things are happening, please give him a normal diet and let his body finish kicking the virus out. Vomiting and diarrhea are the ways the body gets rid of the virus, so don't try to stop those things from happening. Just let his body do it's work and you treat him normally. My guess is, if you do that, within 1-2 days all will be resolved. And by the way, if he asks for something and it comes back up and he asks for it again, go ahead and give it to him. Vomiting itself never killed anybody, and his belly needs to get used to regular food again, so it may very well reject something the first time, but tolerate it just fine the next. I have 4 young children and they've all gone through this and this is how I've dealt with it and it does work. The initial stages of the stomach flu are different, meaning yes, a BRAT diet for the first 48 hours of the illness is appropriate. But after that, if they're obviously feeling better but just have one or two lingering symptoms, for heaven's sake, just treat them normally. Good luck and God bless.