The phrase 'in his back' is a correct and usable part of a sentence in written English. You can use it to describe the location of something relative to a person's back. For example: "The hunter felt a sharp pain in his back as he leaned forward to take aim."
"In his back pocket.
He reported a pain in his back.
"They said it's in his back".
We are in his back yard.
There were strange muscles in his back.
There's a turning key in his back.
Eckert's is usually in his back pocket.
Ken put them in his back pocket.
He was shot in his back".
Two pieces of shrapnel stayed in his back.
Then we sat under an umbrella in his back yard.
Ludwig does not simply clarify my doubts with English writing, it enlightens my writing with new possibilities
Simone Ivan Conte
Software Engineer at Adobe, UK