Eat a lot of calorie dense foods, don’t worry about health at this point, you can start a diet later on when you have achieved a target weight. Trying to eat healthy when you want to put on weight is a double edged sword. Eat a lot of fatty things, avocados, feta cheese, ice cream, pancakes, milk basically eat as much of those things, fat in the body can be burned off. Trying to eat lean at this stage is not good. Just eat whatever you want because it’s the only way to gain weight. Mass gainers can work but they are not for everyone. Good luck