It is more typical of a Shepherd than a Rottweiler, but some dogs refuse to eat more than enough to keep themselves lean. If he will eat more than you are feeding him, you could try feeding a little more, but don't over do it. You still want to be able to feel his ribs as illustrated in the link in the sticky.
You might read up on Rottweilers.
American Kennel Club - akc.org is a good starting place for many dog topics, more reliable than many other sites. I know some lines of Labs are slender and thin, and perhaps Rotts too. Peggy came to me at 7 months with her ribs showing. A couple of months of 7 cups a day of Pro Plan fixed that, but she was still a tall slender dog. At a year old, both her and Lola were a lean 70 pounds. Lola had the typical show looks and only came up to Peggy's shoulder. Maybe I need to dig up a picture of them, scan it and put it up on the web.
Lean is healthy. You may be comparing him to overweight Rottweilers. Lean dogs live longer, and more active in later years, and have fewer joint problems.