Loss of appetite is fairly common during illness. You didn't mention if he had any signs of illness, but illness may sometimes be subtle. If able, you may check his mouth and throat area to see you see redness or ulcers which could explain refusal to take solids.

You appear to be keeping him hydrated at this time. I suggest checking with his pediatrician if symptoms are not significantly better within the next 2-3 days or if they worsen.