Gaining weight with Colitis & similar conditions (IBS etc)

My husband has Ulcerative Colitis, but has been in remission for about 10 years. At first, he lost a lot of weight, and couldn't eat any fiberous food. His main cause of flareups are stress related. He took Asacol for years, but they quit making it and switched him to a different med that is supposed to be the same. He can now eat pretty much anything he wants, but he still doesn't like vegetables and most fruits. He eats cantaloupe and apples on occasion. He eats salads sometimes, and likes hominy. Hominy has the hull removed, so that is a good food for colitis. He stays away from spicy foods, and generally eats meats, potatoes, pastas, sandwiches, etc.
I have IBS, and have found that grains are not good for my body. Wheat especially, will increase inflammation all over my body, affecting my IBS, arthritis, and spinal cord inflammation. Processed sugars do so as well. Most of the time, problem foods I eat have processed wheat and sugar together.

You have a number of foods you have already identified as being problem foods, so if you stay off of them, does your colitis calm down? If not, you might try eliminating wheat and processed sugar to see if that helps.
That leaves you with an even more limited menu, but just try to eat more of the calorie dense foods that you are able to eat.

I see you edited your post to include gluten free as well.
Ensure had the same effect on my mom when she was trying to gain weight. And my grandbaby when the dr gave her pediassure, (same brand as ensure) must be the dairy in it.
Glucerna makes a diabetic shake that doesn't have a lot of sugar and no dairy. My mom could handle those.

Can you eat mashed potatoes? if you can handle a little butter or whole cream in them, you can up the calories. And if you can handle a little cheese, that would help as well. I believe that the fat free cheeses may have less lactose in them. I think I may have heard of low-lactose cheeses, but not sure.

can you have roast beef, pork, chicken, etc? If you can cook them with potatoes and mild gravy, or non dairy sauces, that would be good.
Can you eat eggs? If so, add extra yolks to up the calories, and scramble them with a little butter and maybe add a little cheese if you can tolerate.

Oatmeal, bananas, and rice, are all higher calorie foods that you should be able to tolerate.

Can you handle avocado? if so, it is a higher calorie, healthy fat source. Try introducing it a little at a time, tho, maybe no more than 1/4 or even less for the first day, as it IS a fat source, and too much at once could be hard on your system.

I know it can be frustrating trying to figure out what your body can handle, but hang in there and you should get better in time.
Ask your dr for a referral to a Dietician that can help you find foods as well.