I've been reading my old copies of the runaways and the secret (both by Ruth Thomas) and he loved them. I'm not sure what to try next. I was thinking maybe Jacqueline Wilson as he loved cliffhanger but they seem more aimed at girls and I wasn't sure which ones would be suitable for his age. Cathy Cassidy has written some stuff with boys and girls as the main characters but I think they are a bit too grown up for him.