There is a guinea pig thread in this subforum, here's the link
http://forum.purseblog.com/animalicious/guinea-pig-topic-633929.html
I've owned Guinea Pigs since I was five (now 21), I think they are an excellent pet for younger children, however just a few things to consider- make sure you have a proper cage for your guineas, and emphasise to your son that it cannot be left unattended since you have cats and a dog. Pigs live on average about 5yrs but I've had some that were 8-9yrs old when they passed away. They don't do well in very hot or very cold climates and need plenty of fresh vegies/fruit in their diet (as well as dry food, you can look up on any guinea pig website what fruits etc. are okay).
Make sure you get either 2 boys or 2 girls
They're hardy animals but just teach your son he does need to be gentle, and not to drop/throw them back into their cage.
And if it makes you feel better, when we were breeding guineas, we had- 2 dogs, 2 cats, 17 guineas, 3 birds, 2 frogs and plenty of fish... so you're not alone on the animal front hehe!
Btw, if you have any questions about guineas just PM me or post on this thread!