It sounds like a combination of the normal food changes, and his tummy bug.
They do need less food, for a while, as they get older. I find, up to a year, maybe a year and a half, they eat loads, and are very open-minded. Then they get suspicious of new foods, and don't eat that much anyway. If you work at it, they will start eating a wider range again, but they still aren't that hungry.
My DS1 went through all this, now, at 4, he eats a bit more, and is once more reasonably open-minded. Well, he will say "that looks disgusting, but I will try it". And sometimes he likes it.