P.G.
Your pediatrician is the person to ask about the diarrhea.
The banana and egg thing could be texture. Some foods just don't feel good to some people on their tongues. I CAN'T stand how bananas feel in my mouth. I'll eat banana bread, or smushed banana, but a cut up slice? YUCK! Same with eggs - give me scrambled all day long, but I can't do boiled or sunny side up. Too slimy in my mouth and tastes TOTALLY different than scrambled does.
It's a preference thing, and it's normal. Let him eat it how he likes it. Show him how to mash it himself :) He might like that.