Gaining weight is a factor of quantity of input and absorption (and calories burned, but generally low residue diets aren't commensurate to extensive exercise), so you can still gain weight even on a low residue diet. Lean meats are a great source of calories, as are rice and eggs, and don't generally have absorption issues.
You may want to try to snack on small, healthy items between meals to stop the weight loss as well. Carry around Special-K bars or bananas to snack on (or anything else you enjoy and can tolerate).
I would ask your doc about the need for a low residue diet as well. With the exception of surgical reasons, there really isn't much evidence to support general use of low residue diets in IBD - that's one of those historical taught-it-back-in-the-80s treatments that a lot of docs recommend without solid evidence behind it.
I was put on a low residue diet by my gastro doctor because I have a long stricture in my small intestines. He told me this diet would be pretty much for life because of the stricture. I have researched it and it is recommended with stricturing because you're eating food that is the easiest to digest and it is possible to get all nutrients, although I supplement with a multi vitamin, calcium and vitamin D. This diet beats worrying about obstructions and surgery which is a good possibility without doing a diet like this. I just want to prevent surgery, naturally. My crohn's symptoms (very painful stomach aches, loss of appetite, weight loss, nausea, vomiting, night sweats) stopped after being on Asacol and a low residue diet for about 6 weeks - so it's been very successful for me. I'm not about to mess with success.
"Intestinal Strictures
As with most conditions, conservative treatments are often first recommended for intestinal strictures. Changes to diet in order to avoid foods high in fiber, such as raw carrots, beans, whole grains, dried fruit, nuts, bran and other starchy vegetables may be especially beneficial, as these foods can cause inflammation and buildup in the intestinal tract."
Read more: Treatments for Strictures | eHow.com
http://www.ehow.com/way_5230298_treatments-strictures.html#ixzz2L7lOD0MK
"When there are bowel strictures, obstructions, or diverticulitis, an ingestion of low fibre foods can help you control the problem. High fibre foods can cause a narrowing in the bowel to become blocked, also it can also happen that food particles get stuck in the diverticuli, increasing inflammation. Therefore, maintaining a diet rich in low fibre foods helps fighting back these situations."
This diet is also recommended during a flare:
"When someone suffers from chronic inflammatory bowel diseases, such as Crohn’s or ulcerative colitis, a normal diet is usually used, while a low fibre foods diet is needed during a flare-up. Low fibre foods during a flare may reduce abdominal pain, improving dietary tolerance. Once again, raw fruits, vegetables and whole grains should not be ingested during this time."
http://www.fibrefoods.net/low-fibre-foods