I have done leather work for many years. I would use neatsfoot oil if I were you. Any oil will soften it up, but maybe permanenetly. Be careful not to over do it. You don't want to end up with a limp floppy holster. I would try a few applications of saddle soap first and wear it a few times. If it is still too stiff, use the oil, but use it sparingly and allow several days for each coat of oil to sink in before you decide to apply more. You don't want to ruin a fine product. Usually if the leather is stiff, it means it was made from the best part of the hide. I am impressed with the Bandito, but I will probably make my own.
Roll Tide