Can you put a cabinet or something in front so they can't reach the window?
I know saying no or teaching him doesn't always have good results and nobody wants to wait and see if that works.
I had a rectangle glass coffee table and when kids were there I made a Styrofoam guard I slipped over the edge. Just in case. We as adults would pop a knee on the corners. Notice I said had.
I had 13 windows and 2 sliding doors when I priced windows 16 years ago. We too had single pane in a 1985 home. I decided to get a quote and that idea was shut down before he finished the downstairs.
Normally you get a return in about 10 years.
The windows are not expensive, the labor and install can run double.
As far as in the mean time, try putting something in from so they can't get close, maybe rearrange the furniture, move his interest.
Get creative. 4951