Gorgeous panel and what you've added really enhances it. If your brother is a big guy, you'll want to think of his wingspan and height. I like to take height + 10 inches to wrap under the feet and snuggle under the chin.
I would audition a dark charcoal or black (the darkest shade in the wolves' coats) and do maybe a 3" solid stop border, then the grey (second picture fabric) then piano keys of your colorful scraps, and bind it in dark brown.