Certain rocks,bricks,resin reptile items or fish items etc can work to trim nails.I wouldn't use anything else though other than clipping nails yourself or taking him to a vet who actually knows what they're doing,as a lot of other things can cause serious irritation to delicate paws
Things like sandpaper or nail files for example would be completely out of the question,since as mentioned they would cause serious irritation to paws.