On mobile at the moment so can't copy it over, but this question is specifically in the FAQ, and refers explicitly to the printed firing arcs on the ship base.

Here it is:

Q: When exactly can Backstabber trigger his

ability?

A: Backstabber triggers his ability only if no portion of

his base is inside any of the printed firing arcs on

the defender’s ship token.

He really wanted R2, so I gave it to him. Might have to house rule it next game if he really wants it again.

Playing with your son is one thing, and I think it's great you are. My boys don't have any interest in playing with dad's toys...

But if you were playing with anyone else, R2-D2 on the Falcon is a Massively powerful combo.

SteveO makes a good point. YT has a standard forward firing arc; it simply has an innate Turret ability that allows it to fire its main gun outside of that fire arc.