Meal Plan

Firstly I don't know why any competent doctor would give Motrin for a uc patient.

I seem to do well with salmon
Chicken soup
Cheddar cheese
Cooked carrots
Peeled apples
Baked chicken
Rice
Pasta

Adding weight will not be a fast process but as you heal your system will be able to retain more nutrients.
Add healthy oils (coconut and olive) to as many things as possible for both calories and gut benefits. You can take them by the spoonful too but that can be rough on the throat. Putting coconut oil into smoothies is an easy way to go. I also love butter on gluten-free toast (Udi's is the only one that tastes good to me).

Chicken broth along with anti-biotic free meat really helped me when flaring, good energy and vitamins. Fatty meat is again helpful for slow cooking, makes the collagen break down in the meat for stews or soups. Red meat can be inflammatory but honestly it helped me a lot when gut was awful, now in remission I've cut back quite a bit on red meat.

Potatoes are so important to me due to resistant starch. Easy on the gut and so beneficial. Again I put butter on but no other dairy (my gut does not handle milk or soft cheeses).

Give your gut well-cooked veggies: carrots, green beans, spinach (do a search here on this one, many people feel spinach is a huge benefit). Nothing raw, not even salad.
In fact in the beginning you might want to blend or mash things to give your gut less work but good vitamins.

Stay away from anything with carageenan in the ingredient list. Many ice creams and milks have this in it to thicken them.

I agree with many of the foods listed in zengrrls post but everyone's gut is different. My gut cannot tolerate much nut butter (sadly) and grain-free diet (no rice, wheat or any other grain) helped my gut heal. I also have IBS that shows up when I eat fruit and low-fiber is required so no beans for me (except fresh green beans well cooked).

Good luck. It's not an overnight process but when you see improvement you'll know you are headed in the right direction.