I had a billing agent at Blizzard say this about transferring toons between accounts. "The only thing that matters is the last name on the account and the address." While this is clearly against what is stated in the TOS, she told me this. I was astonished to hear it. To test her theory, I transferred toons from my sons account to my account. Blizzard doesn't know he is my son and the transfer went through. I wouldn't take this as cannon, however. Blizzard support is kind of like the airlines, keep calling back until you get the deal you want :/

Maybe make an RAF account in your name and try to transfer his toon to it. Kind of risky, but maybe worth a shot. Again it depends on how far along the toons you have are.