Take out the dead fish and DO NOT BUY HIM ANOTHE FISH.
I'm guessing its some little gold fish bowl? Their just gonna die again. Thats not a proper habitat for a gold fish.
I think it would be better to start teaching about responsibility and how to take care of things. He can learn about some basic fish care stuff. Its never too early to start.
go to
www.aquariacentral.com and their forums for more help on the proper way to take care of fish.
If this was a proper tank for 2 goldfish I completely apologize. But I would still try to teach him how to take care of them properly, and maybe just take him to the store to get a new one.