What was wrong with the little brother?

It was probably an exaggerated form of pica which is a sort of broad catch-all term for a condition where people eat non-food items. Lots of times it is psychological, sometimes it is due to an actual nutritional deficiency and they are subconsciously trying to find something to correct that deficiency. But in most cases someone with pica has just one or two types of weird stuff they eat (one of the most common is paper towels, they tear off little bits and pieces and eat them).

So given that the kid was eating lots of different stuff I had an alternate theory - maybe he didn't actually have pica, maybe since normal food made him puke he was searching for something abnormal that he could eat and not puke. He's a kid after all, so that might seem completely logical to an 8-year old. We never saw him eat the same thing in more than one scene, which made it seem like a plausible explanation too.