First of all, from looking it up it looks like 6th standard is ages 11-12. There is such a thing as starting too early - you can produce a lot of stress and anxiety in a kid by trying to force them to think about career choices and college admissions before they really have any control over that stuff. Allow your child to enjoy being a child, and revisit this in high school.
I don’t know anything about the Indian higher ed system, but SAT or ACT scores are what’s needed for undergrad. The GRE is a grad school exam and the GMAT is for business school. If English is not your son’s native language, many schools will require the TOEFL. MIT does not require it but strongly recommends it if the student does not speak English at home and/or school. But definitely don’t worry about entrance exams now - way too early.