Hate to break it to you, but if your dog is the way you're saying, I wouldn't push the issue. It's very possible that he is never going to like this other dog in a way that allows you to let them be together.
I have a dog like your male GSD. Mike has been fine with dogs he knew when he was a puppy, but he'll turn 10 in a few weeks so the population of Mike's Friends is shrinking quickly! Mike does not like other dogs, Mike can not be trusted with other dogs. He doesn't start any trouble but he'd rather the other dogs NOT touch him or get in his face. He reacts first and thinks later. That's the way he's wired, and I do NOT have Mike out casually (not under command) with other dogs besides my old girl Ianna. He is fine with Bunny thru kennel fencing or an x-pen, and watching them play thru the fence I could almost convince myself that he'd actually be friendly with Bunny. I'm not fooled, I know what he's like and how quickly he can go off.
Please be very careful with this situation. The impression your dog makes on this baby puppy will stay with that puppy FOR LIFE. A bad experience at a very young age can turn the Rottie into a dog that either hates GSDs or is terribly fearful of them, and neither option is good. It has taken me a very long time to get Bunny comfortable with random GSDs, especially males, after her ONE bad experience with Mike when she was 10 weeks old. She actually does have male GSD friends now, but they are definitely not her preference.
Please listen and learn, and don't take chances with a baby puppy.
--Regina and the GSDs:
UCDX von Sontausen Holy Grail (The BUNNY!) HGH, UD,RA,BN,GN, ASCA UD RAX High in Trial
PAM Monster Mike SchH2,UDX,OM1,TD,RE,VER
PAM FGDCh Ianna von Sontausen UDX,BH,TD,RE,PT (4.26.1998 - 2.11.2013)