Since your son found it in Iowa, as I see it you have three scenarios:
1. It's a natural material instead of being glass. Quartz crystal or something like it. Rare find, and made from a material that doesn't seem to be very common in your area.
2. It's post contact glass, made in a very narrow window of time when natives had access to glass waste and still knapped like that. Really rare find, and that style of stemmed knife wasn't particularly common in the 1800's when it might have been made & used. Worth noting that the common glass that they would have had access to usually wasn't crystal clear. (You tend to see more historic glass knapped items made from brown, amber, greenish clear glass from bottles and other items.)
3. It's modern glass and not old. Not sure how it would have gotten out there, but people have found modern stuff while field hunting. I've heard of some hunters who will occasionally whack out a point while walking.
Option 1 is the most likely, but if you are really certain it's glass (flat spot, bubbles, part of a Coca-Cola logo still there, etc.) then option 3 is more likely in my opinion. In California and other areas of the west you might see glass used longer or much more recently, but settlers cleared Native Americans out of Iowa pretty early on.
