Regarding your possible cancer, IMO, I would go for another opinion. Dr.'s have been known to get it wrong before and one more opinion couldn't hurt. (((((hugs))))) I wish you all the best.
Second, I couldn't believe what I read about your D7 because for the last week or so I have been going through the same thing with my S7!
S7 was never a back talker and always did as I asked, now he has come up with an attitude and not respecting me all the time. My Mom said he is being a typical seven year old where he is testing his limits. It just hurts me for him to treat me that way and I sat him down and told him so.
I basically told him that I loved him very much but I would not tolerate his behavior. I told him that it hurts my feelings when he treats me like that and I don't like it. I have sent him to some time out and eventually he comes out of his attitude but then he will get snotty again. Last night I had him go to bed a little early because I thought he wasn't getting enough sleep.
It just bothers me because I know he does not treat his father like that. Maybe he wants to but he is too scared so he takes it out on me?
Sorry for venting on your thread. I just wanted to let you know you are not alone in that boat. ((((((hugs))))))