And that can be a head's up for food problems.
Low oxalate diet means steering clear of foods that are high in a naturally occurring substance called "oxalates". There are a lot of these kinds of "families" of foods that could irritate your son's system. For my family, it's salicylates. For others, it's nightshades. Not chemicals--they occur in fresh (even organic) foods. And this is where keeping a log becomes pretty critical. For instance when I scan a clients log and see that they react after tomatoes, I know it's probably not oxalates--but could be salicylates, phenols, nightshades... and so starts the process of connecting the dots. But you do best doing this by keeping a detailed log on your son so you can make the process go a lot faster and more effective. I have one if you want it (a PDF you can print out) just pm me.
Oxalates are associated with kidney stones, but truly--your son could be reacting to any of the multiple food families or just 1-2 foods that really bug him.