I need to figure out how to help my mom's cat to lose weight...?

My mom's cat is 4 years old, will be 5 years old in February. So of course, he is no spring chicken. He is fed Purina Sensitive Systems, as he has a VERY sensitive tummy. He can't stomach any other type of food, even when it comes to poultry or tuna. He is very lazy, and sleeps all the time. My mom leaves the bowl out for him. I've tried giving him a schedule-based diet, but he threw a fit even though I persisted for nearly a month and a half. I need to get him to agree with the idea of limited food. Any ideas on how to get him comfortable? Limiting his food intake is a start. I also need him to exercise. He's not much into toys at all. Any ideas for this fat little kitty?