Okay, are you ready for a professional opinion? I am a personal trainer, a sports nutritionist, and a senior fitness instructor, certified by the International Fitness Association.
To put on weight, you must consume more calories than you expend every day. You can calculate your daily calorie requirements using the calculator found at
CALORIE NEEDS FOR MEN. Adding 500 calories a day over what the calculator says you require will result in a gain of 1 pound a week. I do not recommend adding more than 1000 calories a day, which would result in a 2 pound a week gain.
To put on healthy weight, which means lean muscle mass, you must consume the right calories. You must also exercise your muscles to increase their size, which will result in a weight gain. Of course, every added pound of muscle will consume an additional 300 calories a day, so as you build muscle, you must continue to increase your calorie consumption. Oh, and muscle does not weigh more than fat. That's like saying a pound of lead weighs more than a pound of feathers. Muscle is, however, denser than fat, so as a person loses fat and gains muscle, they may look thinner, but could possibly weigh the same, or even more.
Unless you are a marathoner or tri-athelete, I recommend a balance of 40% protein, 30% carbohydrates, and 30% fat. For runners, they can increase their carbohydrate intake to as much as 60%, reducing the percentage of protein and fat in the process.
I recommend lean protein such as the lean beef and pork, turkey, chicken, and fish; low fat or no fat dairy products such as milk, cheese, and yogurt; and low cholesterol egg products such as Egg Beaters or Better 'n Eggs. You can also make high protein drinks with whey protein powder, yogurt, and egg substitute, along with various fruits and/or fruit juices.
I recommend complex carbohydrates from whole grains; high fiber cereals, especially those with soluable fiber such as oatmeal; brown rice; and sweet potatoes. If you MUST eat white potatoes, include the skin, which has most of the vitamins and all of the fiber. Oh, and contrary to what most people believe, corn and peas are not vegetables. They are simple carbohydrates. Avoid them.
I recommend the majority of your fat intake be monounsaturated and polyunsaturated fats, limiting your saturated fat intake to less than 10%, preferably 5%, and eliminating transfats altogether.
So, increase your calorie intake, excercise, and eat the right foods. Oh, one more thing. Do not try to consume those calories in three meals a day. You should be eating at least 6 meals a day, and the first one should be breakfast, within 30 minutes of rising. If you aren't a "breakfast person," you will have to teach yourself to be. Start with something simple, like, say a protein drink or an instant breakfast you mix with milk. Personally, I prefer a large glass of Ovaltine.
The above recommendations will allow you to consume all the extra calories you need and keep your metabolism even throughout the day. Good luck, and if you have any questions, don't be afraid to ask.