Robinogue, you're likely to hear as many diet recommendations here as there are people. Each person seems to respond to different things. High fat foods, spicy foods, dairy, raw fruits and veggies and gas-producing foods (beans, cabbage, etc) are frequent offenders, but not for everyone. I tolerate dairy with no problem, but can't eat raw fruits or veggies. Others can't tolerate dairy at all.
One thing that may help is for him to keep a diary of what he eats. After a while, we tend to notice that every time we eat a certain thing we have trouble, then we can adjust our diets. I don't believe at all that food causes UC, but it certainly can make a flare worse and more painful if we eat the wrong things.
Left-sided UC diagnosed 2001. Taking Colazal and Imuran, Remicade infusions and moving into remission. Finally off steroids after four years!
Now in remission for almost two years. Remicade has been my wonder drug.
"If you spend your time second-guessing your past decisions, you'll never have time to enjoy today."