Hi A. --
At the risk of sounding too salesman-like (!), can I recommend Barefoot Books? Barefoot is an independent children's book publisher, and I started selling these beautiful titles about 2 years ago. You can check out my website at http://lorihachenski.barefootbooks.com.
There are quite a few really vibrantly-illustrated paperback books that come with a singalong CD -- so there are no worries about your son trying to sit through the whole book. He can get up and dance! My kids are 2 and 4, and we like The Animal Boogie, Farmyard Jamboree, and Portside Pirates. If he's really into vehicles, check out Magic Train Ride, We All Go Traveling By, and Journey Home from Grandpa's. All those books are $9.99 each with the CD, or there's a brand new one in hardcover called Driving My Tractor (with the singalong CD) for $16.99
Hope this helps! I'm always buying these books for gifts and for my own children.