I googled "turned his back on" and didn't find a single example used in a literal sense. I suspect that even if it were used with a literal component to the sense, it would still have a strong whiff of its transferred meaning.
Is this sentence correct? Jack's parents are always concerned about how he is doing at school, but Jack often turns his back on them. I am wondering whether turn one's back on is only used to mean refuse to help.
Makes sense to me! I'm sorry, I didn't exactly understand your question.
The sentence could be even clearer if it ran: Jack's parents are always concerned about how he is doing at school, but Jack often turns his back on their offers of help / their pleas for him to try harder / their attempts to get him to talk about his schoolwork.
It's not necessarily "them" that Jack is ignoring, in the logic of the sentence, but their expression of their concern that he's not doing well enough at school.