the 250 ninja is one of the best beginner sports bikes out there, but height is a large factor in choosing a motorcycle. he will get hit by more wind on a small bike because the deflector is smaller.
the only way to know is to ride it.
i couldn't say for certain without knowing his actual size.
And the trucks shouldn't be an issue as long as he is smart and pays attention to them. you can always speed up, unless there is a car in front, and the act of revving up should alert the driver to your presence. i actually modified my bike for this purpose and put a valve on the header so i can open it (with gloves) when i feel the need to let other drivers know I'm there. (this makes it a lot louder) (i just cut a hole and braze welded the valve on where it can't be seen and wont damage my engine)