Coco Coir (eco earth) Pet stores sell it in bricks that you hydrate or you can get some spagum (peat) moss dirt at Lowes (if in US, not sure where in Europe or other places) for 10 bucks for 9 yards. That will last well into his Juvenile stage. LOL...
Anyway, they both hold moisture really well and are mildew resistant if not out right repellant. I have yet to see any in any of my pens...