he's not reducing gravity though, he's reducing mass. Most of his weightless gimmicks hinge on that. He would be a lot slower to move and react in many ways if he literally only controlled gravity.
weight as we feel it comes from the pull effect of gravity accelerating us towards the ground.
we get lighter or heavier depending on how strong we get pulled and how much mass there is to pull.
so by my understanding his magic so far is only amplifying or negating the gravitational pull, not the mass.
i am quite sure of that because he can float in place when lightened and doesn't naturally float upwards when weightless.
i have read another manga that had weight manipulation as the MCs magic and have seen some videos detailing such effects, so i am familiar with the differences of weight and gravity effects.
but i am willing to concede the point that his magic may simply affect more than just gravity, it is a conceptual kind of magic and not strict science after all.