Sorry for posting, but have you ever tried changing your password on your minecraft account?
~ @Toostenheimer
If he can't access the account, then he won't be able to change the password..
Mojangs customer service sucks, They are useless. At the max, all what they will do, is ask you for your transaction number from your purchase do the account, and if you don't have that. You will have to contact Curse, and they will help retrieve your proof of purchase.
There's also the email way, But I'm guessing this is the first thing that everyone does when they can't get on their account. As chrisandmatthew said to go with the "Forgot my password" procedure, this will be the easiest way, although the automated service which deals with email verifications/Support emails also suck, On all of my minecraft accounts, I've has trouble receiving support emails.
Or you could join in and ask people on the Mimecraft Help IRC chat, http://webchat.esper.net/?channels=minecrafthelp, they will help you out, and will thoroughly explain every procedure possible for retrieving your account, plus this is a much quicker way than contacting mojang.