You may want to look into Absorb Plus, a nutritious shake specifically formulated for IBD patients with compromised digestions.http://www.holistichealthshoppe.com/#6
I'd be careful with the cheese suggestion. Many IBDers have problems with dairy. Some other options weight gaining options are nut butters, avacado and supplementing with healthy fats: fish, flax, olive and Udo's. Check your local health food store for protein powders and meal replacement powders. These are healthier choices than Boost and Ensure.
Starting a gentle weight training program is a great way to put healthy weight back on, and it's good for your bones if you're on Prednisone.