That was an opinion. Not a diagnosis. That would be nearly impossible from a few crude photos.
It could be an inflammation. I would treat it as a dermatitis for now, soothing oils and Vetericyn wound spray could produce results if it is.
If this gets better, it's not cancer. If it gets worse...
Opinion is all I can offer since I have never seen such a thing. The color alone is inexplicable. The nodules are unexplainable. Therefore, my opinion is that he has abnormal cell growth going on. We call that sort of thing cancer.