I'm so sorry you've gotten this dreaded diagnosis. I was reading back over your posts, and saw that he'd just had a check-up in May, and had no visible problems other than the teeth. Believe me, there is no way you could have known. Cancer is so insidious, it lurks there, unseen, till when we finally do know, it is sometimes too late. Has the vet done chest X-rays? If there are lung mets, then, yes, it has already spread. I think you'll find that so many of us who have dealt with osteo didn't find out till there was a fracture, or a visible tumor that appeared and grew rapidly, or the dog stumbled, etc. So please don't beat yourself up. I know it's hard, but focus on Cuz right now. If you worry too much or feel unnecessary guilt, it just steals away what time you have left with your boy, and the monster has won again. Live in the moment with him, manage his pain, and follow your heart. You will know what to do, and he will let you know as well.
You are in our prayers.