Kitty has bad gas =X(?

My mom's 2 year old cat has really bad gas. He is currently eating Purina One Chicken & Rice formula. I think she is thinking of switching him to Chicken Soup for the Cat Lover's Soul Adult Lite formula...hes like 14 lbs and she is trying to get his weight down (she just adopted him like 2 months ago). Does anyone have a better suggestion of food to switch him to? Anyone else have this problem? What helped? He's pretty potent. Thanks for any help!