I'm bringing home my first-ever pair of pet rats on Friday, so I agree with Colleen (and everyone else).
If your son wants a small pet, a hamster is probably the worst possible type to get (especially for a child). They are BY FAR the least friendly small pet you can own. And they bite. A lot. And not just when they're scared, like other rodents. By the way, this stuff isn't my opinion, it's info taken directly from my small animal care textbook and lecture notes from 2 different companion animal care/management classes.
- Gerbils are MUCH friendlier and social.
- Guinea pigs very rarely bite -- plus they kinda "talk" to you, which is cute.
- Rats are highly intelligent, social, trainable, and bond strongly to humans.
- Rabbits have very stinky pee (so I wouldn't get one), but they rarely bite.
- Turtles are AWESOME pets (I've had 2)
I'd pass on the hamster, I really would. There are plenty of other small animals that are just as cute and cuddly-looking that are ACTUALLY cuddly

.
You could look into ferrets, sugar-gliders, chinchillas, a de-scented skunk...
(BTW, I also wouldn't get a mouse. They're "biters" and can get fairly stinky.)