There's a ton of people who do the same exact things and have no health issues. For all we know it could be genes.

Hell I even could use myself as an example, I'm also 23 and I usually spend 10h or so sitting in front of a PC each day, this has been going on for years and I have no issues with my vision or legs and I get much less leg exercise than he does that's certain as I barely leave my house.

My mother's sister used to spend a lot of time on PC (Job) but she did a lot of exercising as well (swimming pools etc.) then few years ago she was diagnosed with muscular dystrophy on her right leg so she had to go to doctors 'n shiet, make leg exercises 'n stuff, I don't know full details since I never cared, as far as I'm aware she's doing just fine nowadays.

His lifestyle isn't necessarily the culprit, there's a ton of things that can factor into it.