How to just flash but not actually shoot in Nikon D5000?

Whenever I take a photo my son wants me to take one of him too, I have sometimes end up with my memory card full way too early because of this.

Well you can buy a number of memory cards to start.

They are so inexpensive, most of us have six or more in little cases we carry with our cameras

Or you can just wean him off his photo fixes.

Set the camera to a lower resolution or buy a cheap camera that can get thousands of pictures on a card. Or you can take a picture of your son and delete it when he isn't looking.

Pretty sure the only way to trigger the flash is to take a photo.

shouldn't you be taking pics of your son instead of yourself? But i digress.

just delete the photos afterwards if it bothers you that much. Or get a bigger card or an extra card.

Just learn to say "No".