I don't believe that this is how the referral program works. To my knowledge it works in that when you visit the site using his referral link before/when you initially create your account then you are considered to be his referral. Then whenever you buy premium he gets a month. I don't believe that there is any way for you to become his referral after you create your account. But, you can open an eticket and see if they can do anything for you.