9 months old is part of the gangly stage in Boxers, isn't it? I wouldn't be too worried at this point Cassie. A gain of 5lbs in one month is a lot. You want to go slow...gaining weight too quickly at this age can cause stress to Diesel's joints as well.
It sounds like he's eating an awful large amount of food as well. The largest of ours is about 70 lbs and he only gets a maximum of 1 1/2 pounds raw or, on the occasion Orijen is fed, he only get 1 1/2 cups maximum per day.
What are Diesel's stools like? When, and how often was he de-wormed? Using which product?
"Education is a progressive discovery of our own ignorance." -Will Durant