I am so happy for you and your family. I will just dash off a few "just in case" items that you probably already know! Rottie are sensitive dogs. Never think you have to be harsh in any training. Dogs are very aware of all body language. If you use your hands to signal a sit and then to give him a treat, it will help teach him your hands are a good thing. Keep him on leash with you in the house for a few days and praise when he "goes" outside. (Totally ignore any puddles you find inside) If he does "go" inside--YOU slipped up.
) He will be housetrained a lot quicker if you are fanatically vigilant the first few days. When outside on the street I try to keep my body between "threatening" things and the dog. Naturally, if you can spot "scary" things before he does, a sit and treat should help.
Most importantly
be joyful that he is a friendly puppy!!!! You will find that he will gain confidence and if he is treated kindly and is well trained he will be all the dog you could ever want. Thank you for reading my 2 cents. All the detailed expert advice will be found here at RDN by the experienced trainers, rescue people and breeders! I am just a Rott lover!