You need a diet and workout plan specifically designed for hard gainers/ectomorphs (naturally thin, small-boned, long-limbed) with modifications to the diet for UC. I highly recommend Anthony Ellis' program Gaining Mass. Tall guys like you that started out with an even lower weight have done well on the program. Check this testimonial out:
http://www.musclebuildingcoach.com/public/204.cfm
Basically, you need heavy weight workouts made up of low rep compound moves working each body part very thoroughly but only once a week. Muscle is built in recovery, not in the gym. If you work out too often, you keep breaking muscle down and don't give it a chance to grow.
Your diet needs to be high quality protein, complex carbs and healthy fat. I would recommend protein and meal replacement powders to supplement your diet. Ensure, Boost, etc. are a mistake... loaded with sugar and unhealthy fat. You want to gain muscle, not fat, right? Absorb Plus is a protein powder developed specifically for IBDers. Egg white protein powders are great, too.
Check out the whole program here:
http://musclebuildingcoach.com/
And email me if you want to discuss further. This program took me from a low of about 90 lbs. at my sickest (5'4" female) to a healthy and curvy 119. My "normal" weight prior to getting UC was 105.