Your son's bag clearly didn't fit - width (girth) or length, or both, were the problem. When you buy a bag in a store, always crawl into it, zip it up, cinch the hood, and make sure it fits. (If the store won't let you do this, or the clerks and customers give you funny looks and pull the kiddies closer, you're probably shopping in the wrong store. )

If you order by mail, make sure they allow full-credit returns; then do the same fitting routine you'd do in a store.

As far as side-sleeping in a mummy bag, you don't turn inside the bag. Instead, lying on your back, scoot toward the edge of your pad, then roll you and the bag toward the center of the pad. This means you end up properly oriented in the bag, and you can simply bend your knees, taking the bag with them. It takes a little practice (again, in the store and/or living room), but it's really not too hard.