What is Hayama Akira's ethnicity supposed to be?
As someone who is from Tamil Nadu, I'd say it's very evident that he really is from South India. Was extremely surprised to see Kozhi Varutha Curry as one of his dishes because it's a purely Tamil-based dish. 
Even in one of the pages in the manga where they show him living in the slums, I could see a board at the edge of the panel that had Tamil writing, I think it's enough proof.