We had another rottie in our puppy k class and Max was half his size! It used to bother me a bit since the other owners would be making comments about how small Max was. I eventually got over it though. It's not about how big or small your pup is. As long as he's happy and healthy, there's nothing else that matters. He'll catch up to the "others" soon enough. All puppies grow at different rates, yours might just be taking his time, like Max did. Besides, it's best if they grow slowly anyways. All good things come to those who wait
Typically, a male pup should put on about 10lbs a month, but that's a very loose standard. Max was no where close to 30lbs at 3 mos, but he's now 90lbs and almost 10 mos old.
How big are his mom and dad? He should fall somewhere in between, give or take.