Hello and welcome!!
What to feed is the eternal question. The rule of thumb is that cats are obligate (true) carnivores and need to have a diet that is high protein and low in carbs. Right now he is eating Cat Chow Indoor? Is he eating any wet food at all?
This is the ingredients list for the CC Indoor (from Purina's website). As you can see, there are a lot of fillers in there. These can contribute to a cat's weight gain especially if free fed :
Corn meal, poultry by-product meal, corn gluten meal, soy flour, animal fat preserved with mixed-tocopherols, powdered cellulose, animal liver flavor, soybean hulls, calcium carbonate, phosphoric acid, salt, natural flavor, potassium chloride, choline chloride, parsley flakes, Vitamin E supplement, taurine, zinc sulfate, ferrous sulfate, manganese sulfate, niacin, Vitamin A supplement, calcium pantothenate, copper sulfate, thiamine mononitrate, Red 40, riboflavin supplement, Vitamin B-12 supplement, Yellow 5, Blue 2, pyridoxine hydrochloride, folic acid, Vitamin D-3 supplement, calcium iodate, biotin, menadione sodium bisulfite complex (source of Vitamin K activity), sodium selenite.
I would stop the free feeding and incorporate wet food into his diet. You didn't mention what your budget is but if you're looking for a better quality dry, I recommend Orijen. It's pricey and not available from big box pet stores like Petco and Petsmart but it's good. Most independent pet stores do carry it.
Here's their ingredients list as a comparison
Boneless chicken,* chicken meal, chicken liver,* whole herring,* boneless turkey,* turkey meal, turkey liver,* whole eggs,* boneless walleye,* whole salmon,* chicken heart,* chicken cartilage,* herring meal, salmon meal, chicken liver oil, chicken fat, red lentils, green peas, green lentils, sun-cured alfalfa, kelp, pumpkin,* butternut squash,* spinach greens,* carrots,* apples,* pears,* cranberries,* mixed tocopherols (preservative), chicory root, dandelion root, chamomile, peppermint leaf, ginger root, caraway seeds, turmeric, rose hips, freeze-dried chicken liver, freeze-dried turkey liver, freeze-dried chicken, freeze-dried turkey, dried Enterococcus faecium fermentation product, Choline Chloride, Vitamin A, Vitamin D3, niacin, ribo?avin, folic acid, biotin, Vitamin B12, zinc proteinate, iron proteinate, manganese proteinate, copper proteinate, sodium selenite *delivered fresh and preservative-free
If you're looking for a more budget friendly choice, Merrick Purrfect Bistro is a good choice (Petco carries it)
Deboned Chicken, Chicken Meal, Turkey Meal, Dried Potatoes, Peas, Natural Pork Flavor, Potato Protein, Powdered Cellulose, Chicken Fat(preserved with mixed tocopherols), Natural Chicken Flavor, Sweet Potatoes, Ground Flaxseed, Dried Egg Product, Chicken Liver, Dried Whey Protein Concentrate, Organic Alfalfa, Cranberries, Phosphoric Acid, Flaxseed Oil (source of Omega 3 fatty acids), Salt, Minerals (Zinc Sulfate, Iron Amino Acid Complex, Zinc Amino Acid Complex, Manganese Amino Acid Complex, Copper Amino Acid Complex, Potassium Iodide, Cobalt Amino Acid Complex, Sodium Selenite), Vitamins (Vitamin E Supplement, Vitamin A Acetate, Vitamin B12 Supplement, d-Calcium Pantothenate, Vitamin D3 Supplement, Niacin, Riboflavin Supplement, Biotin, Pyridoxine Hydrochloride, Folic Acid, Thiamine Mononitrate), Choline Chloride, Taurine, Yucca Schidigera Extract, Dried Bacillus coagulans Fermentation Product, Rosemary Extract, Dried Lactobacillus plantarum Fermentation Product, Dried Lactobacillus casei Fermentation Product, Dried Enterococcus faecium Fermentation Product, Dried Lactobacillus acidophilus Fermentation Product
For wet foods, this will depend a lot on what you can afford and what your cat will eat (the dry too). Most cats cannot resist Fancy Feast and Friskies. The pates from both brands are the best choices. I get the Friskies poultry pates in a case of 32 from Wal Mart for $15. Sheba pates are also good as is the 4Health line from Tractor Supply Company. They have a grain free food that is .69 for a 5.5 ounce can. These are just a few. Petco and Petsmart have other brands such as Nutro Natural Choice, Merrick (the pates are well received by many cats) and Nature's Variety Instinct. These are more expensive than the Shebas, FF and Friskies but are good choices.
I would try to change him to a mostly wet diet with some dry as a topper to entice him to eat if he is reluctant to go for the wet. A lot of kibble cats are. Using toppers such as crumbled Pure Bites freeze dried chicken treats is a good idea as well. My cats adore these things and they are a staple in our house. They are much healthier than Temptations and such as they are simply freeze dried chicken breast. All meat.
He's probably not going to like it when the free feeding ends but it's for the best. Adding more play time in will help him shed some pounds too by getting him to be more active. It's also good bonding time
. Do you have a wand toy or something like that for him?? Most cats go crazy for Da Bird and laser pointers. These (and the Pure Bites) can be found on Amazon.
When switching dry foods, it's better to do this gradually to avoid tummy troubles. Here is a guide