I'm sorry to hear you are having problems. I don't know how underweight you are, but have you discussed TPN (Total Parenteral Nutrition) with your doctor? My son got TPN through a PICC line for a month last year, and it helped him gain weight. This was right before his colectomy and ileostomy (15 yrs old at the time).
My son is also very underweight right now (5'9" and 111 lb) and just can't seem to gain. (Got his J pouch end of June.) The nutritionist last week told us he needs more calories to gain weight... really helpful...