How would you deal with a child that refuses to sleep?

This is a legitimate question - I need adive. I know I ask a few questions about him here, but I honestly don't really know what I'm doing. He's not my kid, just a boy I'm looking after because his family are going through a tough situation. My partner and I are doing a good job for the most part, but with things like this, I have no idea what I'm doing.