It is interesting that you are allowed to use a Module C text as a related text. When I did my HSC, we were not allowed to use a prescribed text as a related text, but I suppose that such requirements vary across schools.
The answer to your question depends on the specific text(s) included in "The Boy Behind the Curtain" that you intend to focus on. Themes that are significant in Tim Winton's texts include identity, spirituality and family life. Looking at the list of prescribed Module C texts provided by your teacher, Kate Chopin's "The Awakening" could potentially be suitable as a related text. This novel explores a woman's journey towards autonomy. A mutual theme between this novel and Winton's texts is identity, which suggests that there may be a strong connection between the two texts that could allow for in-depth analysis.
Another text that you may wish to consider is "On Her Knees", which is a short story that happens to also be by Tim Winton (although it is not a prescribed text for HSC English). This text tells the story of Victor Lang and his mother Carol, who cleans houses for a living. Family seems to be a significant theme in this text, which could make it suitable as a related text given the apparent similarity with The Boy Behind the Curtain. Here, there may also be a strong connection between the two texts, potentially creating avenues for in-depth analysis.
I hope this helps!