We haven't weighed Rajah for a month or two, but when he was about 7 months he weighed approx 46lbs. (He's a male Brindle)
We also have a three year and a half year old fawn female and she only weighs about 37lbs. She is VERY small, but she eats a lot, has a high matabolism, and is very active. Anytime we ask the vet about her, she just says that Sammi is definitely on the small size, but that she is healthy for her size. She definitely doesn't want her to lose weight but.... She also mentioned that a lot of boxers are actually overweight...that a lot of people want them to be big, so they feed them more and they end up weighing 70, 80, 90 lbs, which is normally too much and bad for their joints.
I would just monitor the puppy and as long as he's eating enough, and he's active not worry about it too much...