Hi
One of the best substrates is a 50/50 mix of organic top soil and playsand.
Almost anything you put in your enclosure will be eaten eventually.
It is possible to over feed them. But the weeds from your yard, especially dandelion greens and flowers, are the best thing for him.
If you want to learn more about him check out the forum here http://forums.shelledwarriors.co.uk/

Dan