What's his relationship like with his sister? What do you for a living? What have his teachers said about how he's going at school? Sorry if this delving too far but it might shed some light for others who may be dealing/have dealt with similar experiences. It may be something medical, it may be something to do with the marriage break-up (was this a recent thing?) moreso than mum simply going OS? A move overseas (your ex-wife) is a pretty big step...
Don't cough up more than you need to on an anonymous forum full of trolls, but the opening up may prove cathartic...