Me, I would start by thinking in terms of nutrition rather than weight gain: lots of pureed vegetable soups, fruit juice, baked custards, rice, steamed white meat, things like that, instead of foods that are high in fat, sugar and additives.
You said that foods like Boost run straight through you, so I am am wondering if perhaps you have a milk or lactose intolerance. You might have more success by trying a supplement that does not contain milk products, and by switching to soy or rice milk, or goat's milk if you can handle that.
You can also buy a tasteless powder called Polycose that is high in energy and a few essential vitamins / minerals. You can add it to hot drinks, mashed potato, casseroles, soup, whatever, to increase the calorie content of whatever you're eating.
All the best,
Co-Moderator Crohn's Forum. New meds thread