We had floor to ceiling windows in a sun room about 10 feet off the ground, so I can empathize somewhat. We also had 3 wood stoves (they get seriously hot). We did what most people do: we taught the children not to go too close to the stoves, and not to run/play rough near the windows. Twenty-one-month-olds can understand a gentle "no" with redirection. Done consistently, the child will learn not to [lean against/whatever] the window. Though we put screens around the stoves because falls happen, but we did not put any barriers in front of the windows. It worked for us.
A simple (but far from rigidly scientific) experiment here might help with your fears. Lay down on your back about 2 feet from the window. With your feet placed firmly on the window pane, try with all your might to push the window pane out. Depending on the floor's surface and the force you can generate, one of three things will happen: the window panel/pane will pop out, neither you nor the window will move, or (most likely) you will push yourself across the floor (how easily this happens depends on the surface you're laying on.) Generate even more force by enlisting another adult to push with you. The point is, you are large and strong, yet however hard you try, most likely you will not be able to generate enough force to overcome the friction needed to pop the window out (unlike poor Gary Hoy.) Your son is small and not at all strong. If you and another adult can't do it, nothing he is strong enough to do will generate the force needed to overcome the friction keeping the window in place.
However, from working in an Emergency Room, I know things happen that aren't supposed to. A baby gate across the window to the height of 3.5 feet is fine if it helps you to feel that your child is safer for it. Most toddlers can't climb over gates that high, but if they do, they will not be able to exert significant force onto the window pane (or frame) to overcome friction causing it to move.
Most important is your child's safety. Also very important is that you don't generate undue fear in your child. If you go ahead and install a baby gate in front of the window, don't make any kind of fuss about it, e.g. if he's pushing on the gate, etc. Your fears may be real (in which case the gate will help) or they may be irrational (in which case, you will still worry.) It's important not to transfer irrational fears to our children.
You might want to ask for a more scientific explanation on Physics.SE.