Why is his mass the same whether he is earth or on the moon?

A person's mass remains the same whether on Earth or the Moon because mass is a measure of the amount of matter in an object and does not change with location. While weight, which is the force exerted by gravity on that mass, differs due to the varying gravitational pull on the two celestial bodies, mass itself is an intrinsic property. Thus, regardless of where an object is located, its mass remains constant.