Oo, pwede na siya kumain ng isda once the sores in the mouth are healed na, pero mas maganda kung soft pa lang ang preparations mo para hindi ma-irritate ang mouth pa rin habang lumalaki ang bagong tissue, kaya fish congee or well-cooked flaked fish mixed sa soft rice is better than firm fish for now. Best talaga na check with your pedia to be sure the mouth is really ready na, kasi bawat bata naman ang healing time depende sa severity ng infection.