I've heard of the salt water flushing/cleansing which is supposedly good for your colon, but as far as a way to pack on 20 lbs, I don't think that is recommended. I'm sure he was on a Mike Phelps type of caloric intake by consuming something along the lines of 10,000 calories per day along with 400-500 grams of protein per day.
It can be done--albeit pretty tough over the course of a month and a half, but a huge weight gain like that does raise a lot of eyebrows.