You can tell him that air does have weight and that it is approximately 14.7 pounds per square inch at sea level. That's why when we descend to 33 feet in salt water, we are down the equivalent of 2 atmospheres, the one of air and another with the equivalent pressure in water.Amber once bubbled...
That's what I said. I'm a diver and he is not so I KNEW they got lighter but he always insisted that air doesn't weigh anything.