Our little ones are so challenging with their eating. At the moment DD isn't eating anything but fruit for brekky so every morning it's watermelon at the moment. I wish I could give you some more help but I'm basically flying by the seat of my pants and giving her what ever she will eat. She will often go to crehe after eating nothing for breakfast and they will give her something there as like your DS she will eat anything there. I was told that it is up to us as parents to provide our kids with nutritous food but we can't make them eat it - so that is what I'm working off at the moment.
Best of luck darl.