NoteZ Body...

It's kind of difficult to tell just by that picture. I can tell you that my boxer has always been on the skinny side with his ribs showing and he eats about 3+ times a day. Now that he is 1 year and not growing so much he is finally starting to fill out.

Also, what does your vet say?

He is thin, but not abnormally so for a boxer puppy Most are ribby at that age. And thin is very very much better than too fat (for his health).

My precious is the same way. She eats and eats, and never seems to fill totally out.