Sprays can be tricky. Without proper protection like gloves, skin contact with chemicals or plant oils might cause swelling. It's often a mix of things, not just the product.
That swelling might not be from the pesticide itself. Some people have a bad allergic reaction to certain plants like basil or mint. If his hands were touching the plants after they were sprayed, it could have been the plant sap mixed with the chemical that caused it.