Yes, you probably should let the turtle go. However, mortality is high both in captivity and in the wild, so you might consider this an educational opportunity for your son. You will not need an oxygen pump but that would cut down a bit on the smell. A filter might help more. Make sure that your son washes his hands thoroughly after cleaning the tank. Turtles carry Salmonella, which doesn't seem to bother them but can cause nasty diarrhea in humans. Food should be high in protein and calcium, such as earthworms, bait fishes, and liver. preferably dusted in bonemeal.