I think your son might be thinking about a propane
is much heavier than air and will quickly sink to the lowest point in any enclosed space were a leak to occur, therefore for a propane
alarm to be effective it must be placed as low as possible.
CO (carbon monoxide) on the other hand, is virtually the same weight
as air, and should disperse homogeneously throughout an enclosed space. It seems to me waist high should be a sufficient position for it.