My DS is 13 and over the past year it has become more and more noticeable that he has a curve in his back. It’s there all the time but most obvious when he sits on a chair, as his back is almost C shaped! It’s not just me that can see it - other people have commented, and when DS looked at photos of himself sitting he was quite shocked.
I have reminded him many times to think about his posture, but he says sitting straight feels uncomfortable, and he inevitably relapses after about 30 seconds. He has been to the GP, who says there’s no obvious physiological cause, and nothing really we can do except keep reminding him. 
Aside from the fact that reminding isn’t working, excess nagging from me obviously also isn’t ideal for building a positive relationship with a teenager! But I am worried that if we don’t tackle this now, he will end up with an actual hunch - his paternal grandfather has a very noticeable one, and his dad is well on the way to having one too (which he hates), which is why I thought there might be some hereditary cause.
Is there anything else we can try? A back support garment of some kind? Some exercises? Some kind of chair support? Anything?? I hoped the GP might refer him to physio but she didn’t (in fact I got the impression she thought it was a non-issue). Is it worth looking for a private physio? Thanks very much for any advice.