The bark is generally a no-no altho some ppl do use it.
unless its changed daily it can grow mold. bacteria and harbor mites.
Also if they eat it , it can cause intestine blockages that require surgery.
Most of use tile our enclosures or use newspaper.
Id move your guy to a bigger cage soon. If hes not moving around much he prob wont have much of an appetite either.
When you move him, put the smaller tank inside the new cage for a week or so to help with the adjustment.