That photo could be use to justify "Assamese today" only If that represent the majority to every villedge of Assam.
However, I can find some image on the internet that is not the case.
View attachment 118439
They looks so Thai. and so S.E.Asia.
How can I know you do not cherry pick some boy image that suits your mind?
As for your question which race this boy belongs, I would say Dravidian race.
Actually our S.E.A is the mix between Dravidian Race and East Asian Race. Therefore, we are related to Dravidian Race as well. But if all Assamese looks like these 2 girls, then I would say, they should be classified as S.E.Asia than South Asia based on facial feature similiarity.