Doctors say my son is fine but I'm SO worried. When is it too much?

So my 18 month old has had a VERY bad stomach but since Wednesday night. Since then he's only eaten 1 slice of toast, one scrambled egg and about 2 jars of baby food! That's what he usually eats for breakfast but that's ALL he's eaten in going on 5 days now. He's only drinking Gatorade. He looks so skinny because he's lost 2 or 3 pounds (he was very small to begin with). I took him to the hospital Thursday and again last night. I was sure they'd put him on an IV to get some calories in him but they just said he's fine because he can keep the Gatorade down. I really don't know what to do. Gatorade alone doesn't have very many calories and he has no appetite. How long is it ok for him to just not eat??? Any advice/input is appreciated.