S.B.
Why wouldn't he have diarrhea? A cold produces mucous and snot and it's likely he's not that adept at blowing or spitting all that stuff out so some of it's bound to wind up in his tummy.
It's his bodies way of getting rid of things and as long as he's drinking good fluids, you don't need to worry about him not feeling like eating right now.
He'll be okay.
It's no fun and I hope the little guy feels better soon!