12 week old Rottie, only 15lbs..
Our rottie, Roman, will be 12 weeks old at the end of the week.
He only weighs 15 lbs and my fiance is worried that he's going to be a "small" rottweiler.
Everyone keeps asking if he's 9 or 10 weeks. But we've seen his uncles on his mom's side and they are massive!
15lbs is below average for a 12 week old Rottweiler pup, but that doesn't really mean a whole lot in terms of his eventual adult size.
Perhaps he was the runt or smallest puppy (if a 'runt' is perfectly healthy but just smaller than his siblings, he can easily end up just as large, or larger, when adult), or he may not have had the optimum nutrition to begin with (perhaps part of a large litter, or not weaned onto a premium puppy food).... or there are other possibilities.
Adult height and weight, bone size and other conformation characteristics are determined mostly be genes - mainly by the genes of the sire and dam, and to some extent both their sets of parents and grandparents. A puppy will generally mature to be similar in size to his/her parents, with females being more like the momma's size and males like their dad's. However, all puppies are different and genes being what they are, there are no guarantees and an individual puppy may turn out bigger, or smaller, in the end.
Rottweilers were not originally bred to be huge, giant dogs, they're basically a working breed and although they need to be strong and muscular, they should not be too heavy or out of proportion.
Rottie pups take a LONG time to mature, with the not reaching adulthood until around 2 years of age, for the biggest males it can take up to 3 years, so your little guy has plenty of time to grow!
A Rottweiler is much more than just a big dog, and your puppy's size should truly not be your (or your boyfriend's) biggest concern. It's much more important that your puppy is healthy, happy and well trained/socialized. These are loyal, loving, highly intelligent dogs who make fantastic companions and protectors and whose size (or head shape etc. etc.) is not of primary importance in a loving family life.
Don't ever try to make any puppy (particularly a large or giant breed) grow too quickly by overfeeding or supplementing the diet, this can lead to serious, long term bone/joint problems and worse. Make sure you feed the best puppy food and give him proper veterinary care and a reasonable amount of exercise and then just let your pup have the time he needs to grow and develop at his own pace. That way he will have the chance to reach his full potential in every way possible.
You may also want to check out these pages as they have tips, advice and information you may find helpful....
Rottweiler Breed Standard
Hope this helps, best of luck with your Rottweiler pup.