Try Vise Tape, cut two peices. Wrap the first at the base of your thumb, the second overlapping the first and hitting about half way up your nail. The tape is really thin, so you should be able to use it without having to open your thumb hole. If you have to open the hole, it want be much.
Over time, this will get rid of the callous for sure. It will also help with a consistent feel in your thumb hole.
CC
--------------------
Those that can do. Those that can't complain. Pimpin ain't easy, but it's mandatory.
Most things we like, we don't need. Most things we need, we don't like. Don't confuse your likes with your needs.