Fast food and "fatty skin" are high in calories and fat. A person would gain fat if he consumes more calories than what he burns. A "skinny" guy with a high metabolism can gain weight simply by consuming more calories than what he needs. But this would result in increase of fat, which is generally unpopular. In my opinion, it is better off for
a "skinny" guy to pursue a bulking program where he works hard on a few big exercises and eat more over a short period of time, to gain weight. This way, he can achieve significant muscle growth, allowing him to look much better than if he gained fats alone.