It can be frustrating to be thin but some people are that way naturally as you stated with your father. Suggest working with weights to build up lean body muscle weight which will increase your weight (muscle weight more) and you will feel better. At 19 years old you need to eat 2800-3000 calories per day. Some tips for additional calories: On pasta, then add cheese sauces to the pasta instead of plain tomato sauce or plain pasta. Add cheese to vegetables. Add dry milk to oatmeal, mashed potatoes, gravies, creamed vegetables, and creamed soups (add additional protein sources to soups such as meat). Add beans to rice instead plain rice for additional protein. Suggest eating 3 meals a day constituency with drinking high nutrient drinks - Ensure, Boost or make milkshakes (milk, fruit, and ice cream blended) or yogurt smoothies (yogurt, fruit, and ice cream blended) 3 times a day to help you gain weight slowly. You have to constantly eat and drink the shakes daily to gain weight. Hoped that helped you.
My son is very skinny too, (in the 5th percentile of weight for his age). We had an extensive set of tests done for him at the Great Plains Laboratory, and discovered he is genetically low in Apolipoprotein-B (also known as apo-b). The effect of this is that the body has trouble breaking down and using protein, and is typically very thin. He also has low cholesterol as a result of this, causing nervous problems.
If your body isn't using the nutrients you eat, then you are not a whole lot better off than if you hadn't eaten at all, so there are some extra things you can do that help.
Some suggestions that we have found are helping are:
1) Eat an egg every day
2) Take Omega 3 supplements
3) Take a digestive enzyme supplement before each meal to help improve the breakdown of protein and nutrient absorption
4) Chew your food really thoroughly before swallowing to help break down food and improve nutrient absorption
5) Eat small amounts of protein every few hours rather than large amounts
6) Try to eat natural proteins like nuts, beans and chicken. ie. Make a half a dozen drumsticks and eat one for a snack once a day, they are even good cold! Avoid processed food. Processed proteins are "cooked" and even harder for someone like this to use, such as paturized dairy (and your processed KD, as yummy as it is, he he). If your body can't break it down it is going to discard it straight through.
Implementing these changes my son has gained 3 pounds in 3 months and he did not gain 3 pounds in two whole years before that!
Hope some of this helps! Good Luck.