How do I keep DS away from the windows?

[MrsSnape]

I'm worried sick DS is going to fall through one of the upstairs windows. He's obsessed with sitting (or standing!) on the window ledge, everytime I've caught him I shout at him to get down and tell him that if the window gives way he'll fall right through it but it doesn't seem to bother him. I have visions of hearing a loud thud in the back garden and seeing him on the floor surrounded by glass sounds morbid I know but I keep trying to get it through to him that it could happen.

How do I keep him away from the windows?

he's 7 by the way.

I knew someone who actually had bars put on their 3rd story windows (old house) Looking back it was probably a fir hazard.

How about some of that plastic film? How big is each pane?

These are closed windows?
What is the chance that the window will fall out of it's frame? Is it a very old /badly built/badly maintained house??

Ah, a 7yo should listen!

Put a line on the floor and tell him he mustn't cross it? [hopeful]

They are closed windows but the pressure of a kid leaning against it...maybe I'm paranoid?

What worries me more is that his bedroom window is a fire escape with no lock on it so it actually opens like a little door with a flimsy plastic catch on it to stop it from blowing wide open...this is the one I'm most worried about as he could easily fit through it (obviously as its meant for adults to be able to fit through in an emergency)