How long did the food sit in the tank? It could have created ammonia, which is very deadly to fish...
You fed it enough for 2-3 weeks? It doesn't work that way.. When food and waste just build up in the tank; it rots, just like it would sitting out on your counter.
Goldfish should be fed sparingly, once or twice a day.
Do not get your son another fish, do not try and replace it.
If the food was only sitting in the tank for a few hours then it's your sons fault for not cleaning the tank and taking proper care of his fish.
Fish are very delicate creatures, they require both time and dedication. If your son stops feeding his own fish, (not to be rude) then he doesn't deserve the responsibility of owning one.
Sometimes people tend to forget that these are living, breathing creatures just like anyone else.
They don't deserve to just be replaced in hopes of your son won't find out.
If you do end up buying him a new fish make sure to do at least 20-30 minutes of research and find out what diet they need, their water chemistry and tank size. Don't just ask the people at the pet store/aquarium store, they're pretty close to useless.