How old is your ds, madeit?
I would agree with Footle...I would answer what he asks, but not bring it up for no particular reason. I have two daughters conceived via DE (and am a member of DCN) and have always dealt with all life issues on that basis with them, apart from their own conception which I have 'drip fed' age appropriate about since they were tiny. They are 7 and 10 now, and very comfortable with talking about their origins, and most other things too 
I haven't had cancer, but I've had illnesses and ishoos in my youth resulting in my delay in having kids, and this is how I deal with that situation: I will not lie to my kids, but I don't dump info on them either. I find topics pop up naturally eventually (they haven't all, yet, but I know they will) and then it becomes easy to talk - in a contained, comfortable way - about them with the girls.
What I would say is that if you have any left over worries or grief about your own illness, it would be great to talk to someone else about them - that way, your son won't pick up on any unresolved stress and take it on, iyswim.
As for the behaviour stuff...well, it's hard to comment without knowing your son's age. All sounds rather normal for KS1, or younger, and pushing boundaries all the time with a parent? Join the club 