I worked for 15 years as a vet tech, and have lived with pets for as long as I can remember.
Only get a 5-year-old a pet that you like, because it is going to be your responsibility. No matter how sincere their "I'll take care of it" is at the beginning, they just aren't ready for the day-in, day-out responsibility of a pet.
Guinea pigs have to have the bedding in their cages changed several times a week because it becomes soaked with their urine. Plus, rodents have musk glands, and while a guinea pig's "aroma" isn't as strong as, say, a ferret, it is still there. We have had numerous rodent pets, and love them, but we didn't get them until my daughter was in middle school, and old emough to keep up with their maintenance herself.
Reptiles are also high maintenance. They're cold-blooded, so lizards and snakes have to have heat sources in their cages. You can't just stick a lizard in a fish tank and feed it bugs. It will die.
I'd suggest a goldfish. Make him responsible for feeding it every morning before school. If you put it in a fishbowl, the bowl will need to have a 50% water change about once a week. He can do that - just stir up the water, dip out half (along with the crud that you stirred up), and pour more water in. If you get an aquarium with a filter and pump, you'll still need to do partial water changes, but the filter will catch a lot (but not all) of the crud. If you get a filter, get the kind that hangs on the side of the tank, not the kind that goes under the gravel. The kind that hangs on the side of the tank is much eaiser to clean, while cleaning the under-gravel type means disturbing the gravel, and releasing much of the crud back into the water, which defeats the purpose of having a filter.
Puppies chew because they're teething. They are born without teeth, get baby teeth, lose those, and get permanent teeth, just like people. You have to be sure to provide appropriate items for them to chew on. Don't give them old shoes to chew on - they'll think that all shoes, including your good ones, are chew toys. Same with stuffed toys - if your son sleeps with a teddy bear ot other stuffed toy, don't give the dog one to chew on. He may think your son's stuffed toys are ok to chew on.
You don't often see the lost baby teeth, because they usually get swallowed. Don't worry - that's normal for animals. But any dog, regardless of age, that doesn't get sufficient attention and exercise will engage in destructive behavior - chewing, soiling the house, etc.
What breed to get depends a lot on your living situation. You don't want to put a big dog in an apartment, or keep a working dog indoors all the time.
Toy breeds (pocket-size dogs) are generally not good pets for small children. They tend to be high-strung, and don't tolerate the somtimes rough handling that children are prone to.
If you have a big yard, herding dogs make good kid pets.
Border collies and Australian shepherds, for example. They do require frequent brushing, however.
Hunting dogs sucha as labs or golden retrievers are also generally good with kids. Golden retrievers also require frequent brushing, and labs shed heavily during the summer.
If you need a smaller house-size dog, I'd recommend a daschund or rat terrier. Jack Russell terriers (if your son watches "Wishbone," that's what Wishbone is) are very good-natured, but they are also very high-energy dogs - if they don't get lots of chances to run, they become neurotic.
While the spaniel and setter breeds are pretty, they've been bred for looks for so long that many of them have developed personality issues that would make them poor choices for a child's pet.
No matter what kind of dog, you get, basic obedience training is a MUST. Every dog needs to know and respond to "Come, "Heel," "Sit," "Stay," and "Down." It could save their lives. My daughter's dog got out the other day, and was heading for the street, when she yelled, "SIT!" His butt immediately hit the driveway, keeping him from running into the path of an oncoming car.