Probably very little, yeah. Which isn't that hard if you just lumber out to the car and to an office job and back, or something similar.
Though the real secret is probably a couple dozen thousand calories a day, much more than the lack of activity. A hot dog with a bun is about 300, so he's already getting 2,400 before he's done rubbing the sleep out of his eyes, I'd wager. Add in all of the snacks and other **** he shovels down in a day and it's probably really easy to get that fat, sadly.
****phobia: The fear that a man will treat you the way that you treat women.