Thanks Red Lobster :) At its most basic, his power increases or decreases the effective density of mass in the target object or creature, making them more or less susceptible to the gravity of planet Earth. In the beginning, he will not be able to change the density by much, thus he won't be able to change the weight of an object by much. So, I guess he will start off as a kind of buff/debuff character, hampering opponents and aiding allies. For example, he could make an enemy or their weapon heavier, making it harder for them to fight effectively, or lighten an ally, making them less constrained by gravity, enabling them to move faster, jump higher, etc.
Eventually, his powers might develop, though, to where he could potentially make things weightless or crush them to the ground with gravity :)