I'm so sorry that there is nothing you can do for him, sometimes it's best to put them to sleep peacefully without anymore suffering. It's terrible to hear about the tumors and his deteriorating condition and I can't imagine what you must be going through.
He is a handsome little boy and he will be sorely missed.
Viktor and Sonny will be together again and will always be in your heart, and even though you will never hear his beautiful song, I know you'll always remember it fondly.
Rest well and fly high, little Viktor, your auntie will be there until your last