In India, this issue is faced by many children of inter-caste marriages. The general guideline or rule to be understood is, one cannot change the caste just for the sake of seeking the priviliges of the lower caste.
Children assume the caste status of the father except in special situations.One of the special circumstances would be, being raised by mother who belongs to the lower caste. In your case, this would be valid because, you have been nutured by your mother ( hindu-Ezhava).
Please go through this judgement (http://www.livelaw.in/caste-child-born-inter-caste-couple-depend-upon-circumstances-child-brought-ap-hc/) (http://www.lawyersclubindia.com/forum/Can-a-child-inherit-the-caste-of-mother--78748.asp) and similar ones, approach a good lawyer who can give you the current evidences to support your claim and approach the Thesil/Thasildhar or the VAO. If still you are not being granted, then the lawyer can guide you to get legal help after filing a case.
While you are in the process of selecting a lawyer, please ask your mother to have all the relevant documents related to her, absence of your father and death, evidence that you have being brought up by her as per her caste traditions.