I'd probably find him a better school, tell him to start trying as soon as possible, even though he think he'll fail, he will fail somewhere, but better to fail trying doing something he like than something he does not, but at least he knows where his motivation was and not be blind by other stuff.
To let him know something he hates the least might just be the thing he will love to do and wish he could do, there's no such thing as "there might be something better out there", there's no better side where there's a path made specifically for you, plan the future accordingly, don't listen to the family who will always try to lead you down some place else, and to be independent, get out of there as soon as possible.
And tell him to eat better, treat himself better and have some pride as a human being, and believe and keep carrying on with something you think you and everyone else have: talent, as long as you try. Everyone will tell you "no one else but you know the answer", because they don't want to answer you, they don't want to listen to your bullshit, don't expect other people to give you the answer.