Since he's not a pure-bred, I assume you are not planning to breed so what is the use in having papers? Have him neutered and enjoy your little boy!!
One of my Cavaliers came from a puppy mill. She's definitely my husband's dog (we suspect a woman mistreated her) and we have no papers, but there is no doubt that she is a Cavalier. Unlike your baby, she is smaller than normal for the breed. She's also timid and when we first got her she had never set foot on grass OR been allowed to run. It was pitiful. She's still a bit skittish but she's Bubba's baby and that's all that matters!