It is hard to say. For now, what you want to do is keep him lean and avoid excessive weight gain.
There was an extensive discussion of weight in a recent newsletter from a service dog school.
''Obesity is the number one nutritional disease affecting dogs. It's estimated that 25-45% of dogs in the US are obese. Studies have shown that joint and locomotive problems increase by 57%, circulatory problems by 74%, respiratory problems by 52%, skin problems by 40% and cancer by 50% in animals that are overweight.
Large breed dogs that are overweight also are more prone to developing hip dysplasia. Obesity is especially dangerous for young puppies, as their underdeveloped frame cannot support the extra poundage that it must carry.''
The service dog schools have been very successful with a program of an early switch to adult chow slowing growth and reducing joint problems. As much as you want to see him reach his parents size, you need to give him more time so he will have the joints needs to support that weight. I am a little concerned about a 200 pound Rottweiler. I doubt he would pass this test, LongLiveYourDog.com - Life Span Study - Rate Your Dog
What I would do is make your next bag of chow the adult formula. Evaluate him as in that link and adjust what you are feeding as needed, not for fastest growth, but for ideal body condition. He would eat more if you gave it to him? There are dogs that insist on showing their ribs. In that case, leave well enough alone if it isn't too bad. What does your vet say about him? Some dogs turn out to be runts. My Lab, Xanthe, is about 35 pounds at 4 months.