Why can't he go out?

I'm not sure if it was explained in the subtitles in the International release. I only understood the line because I speak the language that was being used in the movie. Luis' father told him (to this effect, this is only form memory) during the scene where his father released the boy Luis from being locked inside the cabinet by his mother, "You know you shouldn't go out of the house right? Because if you go out, you will commit a sin. And we don't want you committing sins right?' A sign that Luis' father also had psychological issues and it wasn't just his mother that wanted to keep him inside the house.

@ali ... Yes we do speak like that (locally called tag-lish or tagalog-english). Our country's main language which is tagalog (or from the river), being the melting pot of asia, has been influenced by several languages like malay, chinese and arabic from our pre-collonization times. We were also collonized by spain and japan so a lot of our words are also borrowed from them. We were also "freed" and "educated" by the Americans of western ideals from both Spanish and Japanese rules hence the natural use of english vocabulary in our non-formal form of speaking/writting in main language/dialect.