my elder daughter just like your boy,
she started to eat fish when she was 7 months old, and suddenly her ears turned very red. The doctor said she was sensitive to fishes, and later we found that she was also sensitive to eggs. We tried again and again, and finally she was able to eat fish after 2 years old, now she don't has any problem to eat fish at all, don't worry,
good luck to your boy and never give up!