I'm so sorry to hear the news PP.
My Keelah was a Rottweiler too. Strange how they are the LAST dog you would consider having as a first dog (I inherited her with my OH James. He has two rotties and I had never even been near a dog before so imagine how much I crapped myself when I walked into his front room and two Rottweilers came trotting up to me). But she was my angel, my complete companion and it killed me when I heard she had cancer.
You did the right thing letting him go - once a dog has terminal cancer, there is nothing you can do and operations and the like just make it worse. You just have to love them and let them go when they are ready to go. Your doggie let you know it was time to go and you did what you had to do for him. That's what you had to do for him in return for the unconditional love and companionship he gave you during his life.
He had a good one and was surrounded by love every minute he was here. I know it hurts and it will do for a while. BUt eventually it will get easier. love and hugs holly.xx