Sorry if this is really well known already and I'm just going to look a bit foolish here, but yesterday I spoke to my mum (a GP) who told me about citrus fruit intolerance in young children. She said that it's quite common, particularly among boys, and it can give them a high followed by a major low. A bit like giving them tartrazine, I suppose. It's not well publicised as there hasn't been much formal research done into it, but most doctors know this happens. My DS (1.4) has been uncharacteristically crazy over the last few days after discovering satsumas and I think this may be what's wrong with him.
Sorry again if you've read through and already know all this, I hope I'm not just showing my total parenting ignorance!