Get in the back (seat)(,son).
Sit in the back (seat)(,son).
A. in is the correct preposition
B. back and back seat are preferable to back of the car, since by context we are talking about the car
C. likewise, its not necessary to say son (context, context)
D. sit tells the son that he will not only be getting in the back, but remaining in the back. But this can also be inferred by context.
I, personally, would most likely say
I need you to get/sit in the back.
And then, depending upon his age or mood, my child would probably ask Why? To which I would explain:
Your uncle is sitting in the front.
We are picking up Sam, and it's her turn for the front.
I've got these papers (sitting) in the front, and I don't want them messed up.
Point to, or quote, the warning tag about the airbag that says "The safest place for children to sit is the back seat."
Because I told you so.