I doubt it.. Cancer is a very uncontrollable thing, unless you've been exposing him to toxic radiation I doubt you caused this.
There's a chance that's it's benign, if so it'll grow at varying rates, sometimes it'll just stop growing and won't bug him anymore.
If it's cancerous, you may (or may not) notice another tumor appear. IMO if you see another tumor on the body, you should think about euthanasia, since at the point it'll be too uncontrollable and obvious that it could spread to his organs.