I have never dealt with this first hand, but it can be treated. I agree with Matt, this needs some attention.
The reason it is getting bigger is because the blood is filling up in the antler just like a bad bruise. And, as Matt said, if it get banged or cut, there will be a lot of bloodflow.
I have seen this treated with rubber bands and pipe clamps. Anything that will act as a turnicate to stop the bloodflow. The antler will be killed for this growing season. But the buck will more easily recover from this. If it continues, it could end up in a serious antler infection and/or maggot infestation.
Take a pipe clamp and put it around the antler at the lowest portion below the injured area. Tighten the clamp as tight as you can get it. This will cause the antler above the clamp to stop growing and dry up. I have also seen this done with small rubber bands that are used for castrating calves. The same concept in each use.