Thanks for responses guys
Alleycat, he's had those 'wear patches' for as long as I can remember, he certainly doesnt have issues climbing over things - maybe just from rubbing against things over time?
I've cleaned his tank and given him new soil - maybe that will help him, but he's gone straight back into his hidey hole - I'll keep him hydrated and weeds available.
Just dipped under 500g today to 499
, he was around 510...