Rob, I've trained people naturally who have gained more than you. You are right, some people don't even gain 15 pounds. Of course, those are the people who have zero clue as to how to train and eat for muscle mass. They think just taking the steroids will magically make them huge and when that doesn't happen, have plenty of excuses.
I'm sorry to see you're not willing to do what it takes to put on more muscle but, if you're satisified with mediocre gains and don't want to gain anymore during the next 3 weeks, it's fine by me.
MJM, yes, drink milk. Preferably low fat or non fat but, if a person is really skinny, nothing wrong with whole milk in that case. Milk has roughly 1 gram of protein/ounce so it's a great and easy way to add a big chunk of extra protein every day.
Good luck,
MaxRep