Gaining

I suggest eating a good amount of pasta, pizza, and cake for a week or two and it should help you get a little belly to make your BF go wild.

Short term goal? Eat anything in any amount that's generally suggested to not eat or eat in moderation only. Chips, Haagen-Dasz ice cream (top calories), fast food (especially stuff like McDo or B.

Anything that looks like a simple carb is good. Will fill up, fatten up, but gets you hungry soon after again.

Try a shot of grenadine in a tall glass. Add a dash of vanilla and almond extracts. Then fill the rest of the glass with heavy cream. Stir in one or two teaspoons of sugar.

Starting from a balanced diet, every extra 500 calories per day you can eat/drink will roughly result in a gain of one pound per week. 1000 extra calories per day for two pounds per week, 2000 extra calories for four pounds and so on. This is subject to metabolism and other factors but it's a good working estimate.

The nutritional information on nearly every food you can buy is available online, or on the packaging. Look up your favourite foods and see how far each would get you if you stocked up on it. You'll probably find a few things you can enjoy a lot of.