Personally, I think unless someone is really nutritionally deprived, ensure is adding gasoline to fire. Among all the other additives and the high sugar content, it has carrageenan in it which is counter-productive to UC patients. There is some evidence that carrageenan degrades the gut lining and causes colitis in mice. I gave it a few tries and it made me feel like crap, though you'll have to give it a shot and see if it helps you.
I invite you to look into some IBD friendly shakes by doing your own research.
Absorb Plus is a nice elemental shake formulated for those with IBD. I would also second the suggestion of eating real food. You could add in whey protein, raw milk, kefir for weight gain.