What ever approach you use, you do NOT want fast weight loss. I may have misunderstood you, perhaps you mean you just need to quickly get your fellow losing
But..the weight loss needs to be gradual. Here is why: article on hepatic lipidosis
it applies in the sections that discuss what can happen when a cat loses too much weight too quickly, the anorexia that can come with not eating (a vicious circle).
I'd love to know what you currently feed, and agree with Sharky that switching to a quality canned food diet (if an option) is one way to deal with this, or using one of the quality lite foods, or simply, as a member recently discovered, switching to a better quality dry w/fewer carbs, and increasing excercise.
Tips to try to increase activity: place food dish up or down a flight of stairs, or up on a counter or stable cat tree. Schedule a daily play session of 10-15 minutes - using a wand toy or doing toss of a favorite cat ball toy for him to chase and bat around.
hope some of the above helps,