I would not worry about your baby being skinny, especially if your vet is not concerned and there are no medical symtoms to indicate any type of problem. It can take up to three years for a boxer to "fill out" as it did with my boy Gunther. My present boy Ramos is 25 at the withers and 69 pounds and believe it or not he looks thin and you can vaguely see his ribs. This is very normal with boxer pups and I wouldn't worry about it, especially if your baby sees the vet. regularly.
Mike
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Kaiser 1989-2000 beautiful flashy fawn boy, waiting at the bridge
Gunther 1999-2004 beautiful and courageous flashy brindle boy, waiting at the bridge
Jacquets Ramos 8 mo old flashy brindle crazyman, s/d in training