This is Dr. Christian. Welcome to JustAnswer. Does he have loose stool that is getting stuck in his haircoat?
This is a very common problem for overweight cats. There's not one thing that will fix it (other than weight loss) but there are a few things you can try. For instance, if he has medium to long hair you can have him shaved around the rectum. It's called a sanitary clip and helps prevent the stool from sticking back there. Another option is to start adding a little canned pumpkin to his food daily or every other day. Most cats will eat it very well when mixed in and it provides a lot of fiber. The fiber helps to facilitate the movement of his stool but also gives him a fuller feeling so that he may not eat as many calories and may help with the weight loss. Metamucil is another option if he won't eat the pumpkin. A 1/4 teaspoon daily or every other day on his food should help. Another option would be to consider a very restricted calorie diet such as Hill's R/D. It is a prescription diet and when given in strict amounts, can help with weight loss.