If a man has incestuous relations with his daughter, and she gives birth to a baby boy, is the boy:
a)The man’s son
b)The man’s grandson, or,
c)Both
Well, the common definitions of the terms themselves assume a non-incestuous relationship, so I would vote for Both.
I shall choose D: Seriously screwed up. 
In reference to the OP, it may depend on who wants to know and why. Technically, I suppose the answer is “C”. However, in legal circles the definition may be varied. It might all come down to how the parties involved choose to deal with the situation.
One of my best friends is actual her father’s daughter and granddaughter simultaneously (just like the OP, but female). However, that situation is a little more complex. Seeing as how this is GQ, I won’t explain the story but she has always considered him to be her father.
Ask Woody Allen.
BWAHAHAHAHA!!!
Oh you nasty person, Annie Xmas…