Booze impairs your central nervous system which burns a lot of calories passively. I also wouldn't be surprised if he's dealing with some pretty severe muscular dystrophy from a combination of being a lazy sack of shit for the last 3 years and the echo effect of impaired motor skills preventing him from really moving at all. Muscle burns a shit load of calories passively and he probably doesn't move around enough to get tree trunk legs that a lot of people his size tend to have.
Ethanol is also insanely calorie rich. A 26 of whiskey comes loaded with around 1600 calories, and the higher the proof the more it has. I don't know how bad his drinking really is, but even if he's just drinking a bottle a day (unlikely), that's a huge chunk of calories as a baseline, and every trashburger you eat and coke you drink only stacks up from there. Combine that with the fact that his resting metabolic rate is already near rock bottom from his lack of lean body mass, and the other complications added from him getting piss drunk every single night, it's incredibly easy to put on a shit ton of weight with no chance of losing any of it.
To top it all off, he's also quite short. 20 pounds of fat looks a lot different on someone who's 5'6 than it does on someone who's 6'0.