Welcome to HealingWell, Waterdog. I'm sorry your husband is having such a difficult time right now.
Food makes a difference when in a flare, because some foods are just naturally more irritating to the colon. Most of us avoid high fiber, gas-forming, fatty and spicy foods when in flare. Some people are bothered by caffeine, dairy or gluten-containing foods while others are not. There's no scientific evidence that food causes or eases flares, but some people do find that it has an effect for them. We each have to figure out what works for us. Those with UC do have higher incidence of lactose and gluten intolerance, so avoid dairy and gluten-containing foods all the time.
You might try keeping a diet diary for a while. If you start seeing that every time he eats a certain thing he gets worse, then he can think about whether he wants to eliminate that from his diet.
And his doctor is exactly right that Prednisone is not appropriate for long-term use. It can have some horrendous side effects long-term - osteoporosis, necrosis of bone in certain joints, cataracts, diabetes, and more. Talk to his doctor about increasing his medication. Also it may help to get him on Rowasa, which is mesalamine in enema form. I believe that's also a fairly low dose of 6-MP.
If you don't feel confidence in the GI who is treating him, than a second opinion is certainly in order.