Personally I think clumping is better - easier to clean, less messy and goes further But litter is down to owner preference just as much as the cat's preference!
This is the stuff I've always got on best with:
http://www.petsathome.com/shop/en/pe...t-litter-10ltr
Or otherwise I've bought the normal non-clumping Catsan stuff.
If Bertie doesn't like the wood pellets you could always do a bit of a mix of the old and new stuff and reduce the mix percentage until he gets used to 100% wood pellets.