How long have you had it?
My experience with leather has always been the opposite (saddles, bridles, and chaps). I had to oil them every so often to keep them supal. Between oilings (Neatsfoot Oil -nonpetroleum based) they would become dry and stiff.
Your holster may have been absorbing gun oil (petrolieum based oil) from the weapon and that is why it has gone limp. You may try taking a piece of wood and wittling it to the shape of you weapon, then stick it in the holster and lay it up for a spell to dry out. The wood shape will act as a mold. Petroleum based lubs tend to cause leather to rot.
If you can find a custom saddle maker (they are rare) they might be able to tell you something that will help.