Not trying to shoot down some of the advice of some people above, just expressing my concern over some of suggestions, it seems some people are just suggesting you load up on foods high in fat to gain weight, but as with everyone trying to lose weight as well, fat is no the sole cause of you gaining fat. For some reason most of the Americas has this association with fat on the label=fat on them, which is quite ridicoulous since what it really comes down to is overal calorie intake. Carbs, Protein and Fat are all broken down into calories when your body uses it for fuel, so if you eat low fat but make up for that in carbs and protein, well guess what, 2000 calories whether it be made up of 30% fat or 0% fat is still 2000 calories.
My suggestion for you, coming from someone who has had to adapt their own diet to their metabolic needs (i am on a 4500-5000 calorie/day diet to keep my weight maintained) is to make your sure you stick with a proper ratio, i follow the 1-2-3 rule (1 part fat, 2 part protein, 3 part carbs, or in percentages 16.6% fat, 33.3% protein, 50% carb, therefore 16.6% of your total calories should come from fats, and so on) when i am eating. And as far as fast food goes, please avoid it, it may make lots of other ppl gain weight, but those people are gaining weight on garbage food that has no nutrients to it at all, and if you are going to try and throw on weight, i dont suggest you do it in a fashion that is going to lead to you having a stroke or heart attack in 10 years from your arterys being clogged by bad fats (yes, some fats are actually good for you, some are not), besides, as far as i know, fast food is a killer for a lot of ppls digestive systems when it comes to their crohns. If you have any questions about diet and such let me know, its taken me about a year to fine tune mine after a great amoutn of research but now i have mine down to a T, in fact in February lone i gained about 6 lbs of basically pure muscle, something i havent been able to do since i was diagnosed a little over 2 years ago...
PS on a side not, my diet DOES NOT include any forms of High Calorie drinks such as Ensure...