I'm sorry to hear about Butkus. My Little Guy's twin we lost to lymphoma. It is heartbreaking.
I think Rottie will need lots of extra attention and hugs and kisses, and some time to adjust. If he doesn't relax with this, you may consider a puppy friend.
One thing I remember about dogs...they are good about welcoming another..
Now the extra hugs and kisses go to Butkus. I don't know how long the vet told you but as long as you have quality time, that is a blessing, and it sounds like you have bought him time.
My prayers are for Butkus to have many more days of quality time with all of you.
Hugs