This is probably the most frequently asked question in the health forums

and the answer is that it's completely normal for boxer puppies to be skinny like that. As long as your puppy is showing no other signs of illness and has been given a clean bill of health by a vet, you should not be concerned about her weight.
In a mature adult boxer, you should be able to see the last few ribs when the dog is standing normally; in puppies however you will see A LOT more, especially when they're between 2 and 7 months old. Caney was skin and bones as a puppy -- people thought we starved her :o but she had a great appetite, a normal energy level for a puppy her age, and she was healthy. Finally around a year old she started filling out.
The important thing right now is to not overfeed in an effort to make him gain weight. Overfeeding a large breed puppy can actually cause serious problems with bone and joint development. Feed no more than the recommended amount that's printed on the bag of kibble.