Also why does he have to do Islam why not Buddhism
Inheritance of religion, probably. If his parents, grandparents, etc. were/are Muslim, he'd be labeled Muslim from birth and therefore an apostate if he tried to convert to another religion. In fundamentalist Islam, apostasy is considered to be one of the worst crimes a person can commit.
It's bullshit, of course, but if Indonesia adheres to those laws, he would be legally barred from being anything other than Muslim.
Welcome to the wonderful world of state-mandated religions.