In my opinion, both ethnicity and skin color have exceedingly little to do with whether or not you can be considered a whiteboi. Honestly, "whiteboi" is a kinky self-descriptor requiring only that you identify with it for it to be accurate.
BNWO erotic media has an inherently exclusionary bias, and I personally frown upon attempts to further that exclusion in ways that aren't fun parts of the kink. I've even seen particularly dark-skinned black sissies into BNWO porn who label themselves whitebois - and I think that's great! The more the merrier!
You could be the darkest South Asian to ever live and so long as you like the term whiteboi, I think it's absolutely for you!