OK, Kill's estimate of muscle gains may be off, but there are not unheard of. 2-3 pounds per month is about as best an average guy will do. Keep in mind the average male is about 5'9" and about 170, not 6'5" 280-300+ pounds. Also these kids are in the anabolic prime of their lives, their testosterone levels will never be higher than they are at this age.
I wouldn't be surprised to see a kid that's 6.5 feet tall and 280, put on 30 pounds of muscle in 8-9 months. A lot of that also depends on their previous lifting experience. Kids that have been lifting for quite a while should see less gains because once your body is trained you get diminishing/slowing returns. If a kid wasn't a big lifter and then starts lifting and is the size of OL, then they could easily pack on 5-6 pounds per month for the first few months.
Another thing to consider is that some of the weight gained will almost always be fat and bone gets more dense with lifting and added muscle mass so you will see more bone weight as well. So a 30 pound gain with good nutrition and lifting routines will probably net about 22 pounds of muscle and 8 pounds of fat/bone/connective tissue.
Oh.... and I saw Pirsig a month or so ago and he looks significantly more solid than I remember seeing him in the HS season.