I'm sorry to say this but you can't change the last name on an account. Well, maybe if you tell them you married someone but then you will most likely need to send all the papers saying you're really married, legit ID and all that. You might be able to fake an ID but most likely not the paper and you wouldn't want to as if they figure out you're lying you will most likely get into trouble.
About getting the SQ, it's not very easy either. What you could do is get his lastname by talking to him again, saying you're giving the account back or whatever but you need his last name, milk it from him. When you got his last name you buy a new copy of World of Warcraft and The Burning Crusade, creat an account in some random name and his last name, transfer the character to your account and get the scammed account banned.
By the way, you might be able to figure out the SQ the same way as the Lastname, milk it from him or if the question is "What is your favourite color" just guess, there's not to many.
I hope this helps you out.
- Jesus