A novella from I believe the 60s or 70s where an alien named Grey (Gray?) protects a human child from other aliens on an occupied Earth. The two are brought together after Grey's family are slaughtered and the boy's family rescues him in space. Subsequently the boy's parents are killed and the two form a family unit on an occupied and reduced Earth.
One day while exploring the boy finds what he thinks is a dragon's egg....it turns out to be more or less exactly that but it has been sent to kill Grey by other malevolent aliens. Forewarned by the boy Grey is able to kill the dragon when it hatches.
The two live in a hostile environment where the Earth has been overrun by different alien races; many of whom hold antipathy toward each other and humans. Unknown to anyone other that Grey the boy can understand a universal alien language and thus understands that humans are regarded as inferior by many of the alien races.
It is a sweet story of the importance of family and supporting each other. It has stuck with me over the years as I have made my own journey of becoming a father with my own family. I would love to find the name and author and re-read it and seek out any other works by the writer.