Leah, Big is a dead ringer for my Bo!! Except for the size, and no tail, they could be related....Bo is only 8 pounds.
Sounds like Big has Bo's sweet nature, wanting belly rubs, scratches and lots of attention! Purebred male Manx can get to twenty pounds in size, but they are usually short-bodied and stocky, not long (because of their compressed vertebrae).
Lovely picture of your son and the cat, I need to get moving and scan in the pics I have of Bo and my husband (she's a Daddy's Girl big time!)