How can I preserve or restore WWI Gas Mask?

I recently got my great grandfathers WWI Gas Mask from my 65yr old 2nd cousin, it's been in the family basement since 1919. I know it was used alot, my great grandfather was in 58th Infantry,4th Infantry Div and was also a POW. It is dry, it has no holes or cracks, but is really dry. Just wondering if theres anything you can do to make it soften up and make it more flexible without damaging it. I don't want to wear it, just want to keep it in as best shape as I can to put with his uniform,rifle,gear,medals,papers ect in my collection of his. Also the gl is all there, no cracks. It has serial numbers on front, and sides.