Can you overfeed a fish ?

Yes, you can. And it will kill them. Feed the fish as much as it can eat in less than 1 minute once a day. Don't worry; you can't underfeed a fish.

Not to mention that if the food doesn't kill it then the extra poop will.

Yes, you can hurt the fish by overfeeding them. And you can change the water quality too so that it becomes harmful to the fish.

The problem with overfeeding is not that the fish eats too much, but that the uneaten food deteriorates and fouls the tank. That is unlikely to happen in three days, but if the tank is not well filtered your son may have a cleaning job when he comes home.

Just make sure the tank stays clean because any food that is uneaten by fish can become toxic in great amounts. So I suggest having a good filter (that is good for the size of your tank) and cleaning the tank based on how it looks each week (I've read that many people clean theirs out weekly but it depends on the size of your tank and how fast it gets dirty). Be careful, the smaller the tank/container, the easier it is for waste to build up.

Noooo if a fish doesn't want to eat then he/she wont. Just like you will refuse food if you were full as a tick

Yes and the fish will die; some people are just morons.