You need to change your cat's diet. Obesity is a very serious problem in cats, it leads to diabetes and heart failure, and your cat is prone to hepatatic lipidosis (fatty liver disease)also, if he should stop eating for a few days.
Adding a probiotic to his diet daily will help with his digestion. When poop smells bad it is because he is not digesting his food properly. The food you are feeding him now is full of grains and other things a cat's body cannot digest. That's why you need to put him on a better, canned, diet.
First thing to do is take him to the vet for a check up and accurate weight.
Don't let the vet sell you "diet" food. These foods are just more of the same, carb laden junk.
Buy a baby scale to monitor your cat's weight loss. It is very dangerous for a cat to lose weight too fast. No more than one fourth pound a week.
Start your cat on a meal schedule. The goal is to feed him three times a day. He may need more frequent meals as he adjusts to being feed on a schedule.
Measure out how much food he should have for the entire day, and feed from this amount so you know how much he is getting every day.
Begin to transition him to a wet diet. Choose canned foods that are high in meat and low in carbohydrates. Start with only one meal a day of canned food, gradually, over a period of several weeks, work up to getting him on an all canned food diet.
This will help with his weight loss, his general health, and his feces odor.
Cats are obligate carnivores and dry food is very bad for them. Here is a good website that can help you understand more about why a wet diet is better for your cat.