Our 21 month old female, Laila, is very lean and sometimes you can see her ribs a bit. Not her spine though. I sometimes wonder if she's undernourished because she doesn't have that square build you see on a lot of Boxers. But on her last visit to the vet she was 62.8 pounds. And she has no eating problems, so I figure that's just the way she's built. Plus I've read that Boxers don't completely fill out until they are around 2 years old.
Boxers get their full height at 2 years and their full weight at 4 years. My male 6 year old Boxer is 95 pounds and he is not overweight for his size.
I have a 2 year old male and he is skinny. I have tried feedin him more and he is slowly getting bigger. The vet said this can be normal. Maybe just keep an eye on any loss of weight.