Confronting the neighbour sounds a bit combative, maybe instead try to befriend them. You could then get a better idea of how well they're looking after him and offer them your advice about what he needs.
Sometimes families get a cat for a child, who then loses interest, leaving no one who really cares or pays attention to the cat to pick up on his needs. But on the other hand, they may well love him, and think they are looking after him well.