I'm concerned about, "I have dewormed him so many times even though the vet told me he is fine.....".
Why would you introduce medicine into your pup when the vet says he is fine? What's the point in going to the vet when you don't follow their advice?
Unneccessary and excessive use of medications do nothing good for a dog or human, and may be harmful.
If Foreman is eating and drinking well, having no problems with his eliminations, is active and alert, showing no negative symptoms, and the vet has given him a clean bill of health, relax and enjoy him.
Dogs, like all other animals, including humans, come in all shapes and sizes. Some gain weight by looking a pictures of food, others can empty a banquet table down their gullet and not gain an ounce.
I believe reading that it is "normal" in the "appropriate weight" Boxer for the last three ribs to be visible. However, normal or average does not mean that any variation from this is a symptom of problems.
Indy has always been a "lean" dog, well muscled, but hardly any body fat. His sister/cousin Ava definitely has a thicker layer of fat, which may be due in part because she is a female.
Most of us are paranoid when it comes to our dogs, always thinking the worst if they hiccup, sneeze, or cough. Most of the time, the problem exists only in our minds or imaginations.
Better to err that way than to neglect our friends.
Tom