What Can Help My Overweight Cat Lose Weight?

I have a 10 yr old overweight cat. He is a large cat, I would say about 24"'s long and he weighs 28 lbs. He is the only overweight cat of three in the house. He is not very active. I have tried restricting his food over a two year period, changed to weight management type of dry food, fed him only canned food, etc. Over the years he has been stressed each time I travel and leave him. He tends to be very shy, so I think anxiety plays a role. He has recently begun to cough so I'm concerned because I know overweight cats can get asthma. I am looking for suggestions, maybe supplements to help him loose weight.