Dear Maash,
It shows your maturity that you are trying to find a balanced solution to the problem. So I assume that your love for the guy has a solid basis. That I think is a basis on which you can convince your mother.That is if a guy is loved in spite of a sense of barrier among castes by some one who displays maturity, then he should possess some really attractive qualities. You can highlight those qualities that attracted you to him and also put matters in the perspective of current times where inter caste marriage is no longer a taboo. I am sure if you put across the message in the right way your mother will be convinced.
Generally if you are arguing on solid grounds and no emotional aspects involved which earlier used to be the taboo, I think it is not really that difficult to convince people.
All the best
Thank you Srinivasan sir. I will do accordingly.
-Maash