It’s really very simple, his mother married an Indonesian Muslim, in Indonesian law there is no such thing as “mixed marriages”, both spouses must be the same religion so one must convert to the other’s religion, Christian to Muslim or vice versa, so long as both are the same at the time of their marriage.
Therefore even if his mother hadn’t already converted to Islam at the time of her first marriage, by the time of her second she almost certainly had.
Now the child of two people recognised as Muslim under Indonesian law is also a Muslim, end of story. So when little Barry was registered at school in Jakarta he was a Muslim, he certainly was not a Catholic, that’s for certain.
He may well not have practised Islam very much (though I doubt that, any Muslim child in Indonesia would have taken part in the Idul Fitri celebrations), he may have since renounced the childhood Islam which had been created for him by his parents, he may now be a devout Christian, frankly I couldn’t care less, but the fact reamins that he WAS a Muslim at one stage in his life.
To state otherwise is simply to tell an untruth.
I really doubt there was time for Islam in the family life.