Ken Evans 
Anon in Extremis [EPUB ebook] 

The story is narrated by a schoolteacher, about one of his ex pupils, who runs away from his family because he wants to become a guerrilla fighter in the middle east, but the boy returns to the UK, and is arrested , and the Teacher becomes his helper. The Teacher teaches classics and is able to relate to the boys enthusiasms, which are Video Games and wishes to become a mature person. Finally the Teacher is unable to explain why the boy ran away to join a militia group.