My name is Abhijith S. Raj.
So is my surname S. Raj or just Raj?
(Note that my father's name is Sreeraj and that is why the S. Raj.)
Is you father's name '****** S Raj' ? In that case, your surname would be 'S Raj'. However, when a person refers you, he wouldn't call you 'Mr. S Raj' right ? He would just call you 'Mr. Raj'. I'm an Indian and I too have the same problem. But I retained only my last name as my surname to fill out forms and I often get referred as just 'Mr. Nair' on my emails. So to avoid any confusion, you should use your surname as Raj.