I'd probably try rescuing an adult dog over getting a puppy if you're looking for something that will be calm. You honestly never know with pups..my cousins bought a beagle pup who was sweet and cuddly at the breeders but became a demon once they got him home!!!

And I'd honestly recommend a bulldog over a Basset Hound. They're big couch potatoes and are usually total lovers! Plus who doesn't love those goofy faces!!!

Basset hounds were bred to hunt. I personally can't handle the howling..or the ear problems they can be prone to..not to mention I have met some pretty crazy ones!

The Basset Hound is a great breed.

Other calm breeds:

English Bulldog

Whippet

There aren't that many breeds that are calm/quiet that fit into this size range. Mostly the giant breeds are the quiet ones...

You could also look for an adult dog from a shelter, there are lots of dogs of many breeds and mixes of breeds that could suit your needs, as every dog is different. When you adopt an adult dog, you get to skip a lot of the problems and guess work.

NO puppy is laid back unless it is sick. If you get a dog at all, get an adult. Go to the Humane Society or a rescue and look at getting an older, sedate dog that was raised around little kids. (Don't take your son with you--he'll fall in love with an inappropriate dog and wail when he can't take it home.) After you pick out a likely candidate or two, THEN you take son to meet him/her and see how they react to each other.

All "puppies" are hyper... and my dog's classmate was a basset and more hyper than him (he's a lab and known for 3-4 years of hyper)...

BullDogs are the laziest breed, known for being a couch potatoes if that helps