I regularly notice a loved one's bad posture while they are using their computer. It's at the point where I am concerned about the long term health of their back and spine. What would be a good way to bring it up? I have a feeling simply pointing it out or telling them to straighten up won't lead to any noticeable results.
As someone that spend a lot of time on the screen and have an horrible posture, I want to say that it is not that simple to realize that you have a bad posture, especially when you're young (because you may just not care).
What hurted me was picture taken from the side by my mom, she showed me how bad I was stading and how bad I was looking. No one wants to looks like a "?".
Taking picture of someone, even a loved one can be invasive, so I think that you should tell him that you noticed that he has a bad posture and ask him if he is okay to be photograped.
Then, tell him what he can do to improve his posture because showing a problem without proposing any solution won't help. You can say that a physiotherapist or an ergonomic chair may help.
Just said that you care about him, let him know that you will be there for him, that because you love him, you don't want him to suffer in any manner.