"Skinny" or underweight for a snake would be indicated by the backbone protruding a bit and the skin looking loose. This usually becomes apparent in the lower 1/3 first. In severe cases, the skin actually forms folds along the sides. When a snake looses body mass and shrinks, the skin does not. If his skin is a tight fit and there's no sign of a protruding backbone, he's probably not starving to death.