When we are talking about food, we always say 'Pescado', even if there are millons of them in the plate. Then, if you're talking about a boy who's eating fish, then you always say: El niño come pescado.
If he is eating the fishes raw, then 'El niño come los peces'. We have a diferent word for raw fish. Pez in singular and peces in plural. Pescado is once we catch it :)
Hope I was clear enough (and please understand this is much more about vocabulary than about a gramatical rule).
REF.: I'm a Certified Spanish Teacher.