Rather than Ensure or Boost, I'd recommend you try Absorb Plus which is predigested and is formulated specifically for IBDers.
Are you currently flaring? Anything you can do to help heal your gut and thereby improve your ability to digest and assimilate what you're eating will be helpful. Omega-3s will help reduce inflammation. I've had success with L-glutamine, Mucosaheal and George's Roadrunner Aloe Vera for healing.
A diet high in calories, protein and healthy fats along with starting a weight training program will help you gain healthy weight (muscle) as opposed to increasing bodyfat.
At my lowest, I was 90 lbs. at 5' 4". Now I weigh around 115 with 105 being my "normal" weight prior to my UC diagnosis.