He would not need to hold it in the water for long at all. It is extremely easy to cause a chick to aspirate water-just dipping its beak past the nostrils will do the trick if it inhales. Anyone who has needed to use an eyedropper to give a liquid to a weak chick is familiar with this. I have myself accidentally dipped a squirming chick a bit too deep in weak chick solution. If he saw an adult dip the chicks' beaks when they were put into the brooder, he may have gotten the idea to "help" it drink. I am sure all the humans involved feel bad enough without holding an inquest. If you want to get another chick, it will be just fine added to the bunch, even just one, so long as they are still very young. People who are five years old tend to make bad decisions-if all the little ones now know to wait for a grown-up to help them visit with the chicks, I think I'd call it a day....Sad. Very sad. I would not get anymore until he understands how to handle animals. Going out and buying more is not teaching him anything. You need to sit down and talk with him. My six year old knows not to touch the chicks without asking mommy and having me around to monitor. He is good with them and knows they are not -toys- and to be gentle. I'm not sure why your kid would try and force the chick to drink water, or how long he held its face in the water to have it drown...
Sorry if I'm harsh but I wouldn't get anymore or have him handle them without you there. At all.