The main thing that will be holding back your cat from losing weight is lack of exercise. Cats need to do some running about, this coupled with less food will get the cat to lose weight. As its an indoor cat this wont be easy, the main thing you can do is to get some toys and get it chasing them. This will mean that you need to keep it interested so spend time each night playing.
Laser pointers and peacock feathers make great toys that don't encourage your cat to think of your hands as toys.
Walk him around the block daily
Take him to the park that's close and lit him go wild and you can play games and enjoy laying with your cat