E.P.
Could be that he is teething or dealing with some kind of digestive upset or he's reached the stage in his development that he is more aware of different food textures and, now, has decided that there are some that he dislikes. . It sounds like the foods that HE does eat are pretty healthy foods. I would continue to offer him variety but, more importantly, build upon what he is currently eating. (i.e. w/ oatmeal - add some blueberries, yogurt - make a fruit bowl and additional fruit - try smoothies w/ a little juice.) .
Unless it's home-made mac & cheese, the box- kind can be hard on some people's digestive system. My husband always complains if we make it for lunch - rarely will he eat any and when he has, he has felt ill just an hour later.
Regarding any food allergy - are you noticing that he is fussy an hour or two after he has eaten or has a bloated stomach? Or any of the other signs that the previous post mentioned?
Also,regarding food allergies: One symptom that gets overlooked w/ food allergies are bright, red lips. One of my best friends shared this with me - her young son has recently been diagnosed with a severe egg allergy. He always turned down ice cream and custards while her other kids gobbled them up. The dr. suspected an egg allergy and her response was, "he never appeared to have a hard time breathing - in fact - his cheeks and lips were rosy red". Just keep an eye out for that, as well as any other symptoms associated with food allergies.
That's so wonderful that you take care of these children. You must be a very special mom to do this!!!