My cat had some issues using a litter box when I got her from the pound. She was declawed so I can't imagine her previous owner didn't litter box train her but she was year old and had no interest peeing in the box. I took 4 months of trying and trying, finally we figured out she would use Swheat Scoop Litter. It has no smell unlike the pine so maybe that is why she finally started using it? She seemed to despise the pine litter, recycled newspaper type and the clay types as well. She also seemed to take to having a larger box but not one of those hooded ones. I agree his biggest worry might be stress right now. Good luck with your new addition and you are a wonderful person for taking him in!
Amanda
2011 Specialized Epic Comp 29er | Specialized Phenom | "Marie Laveau"
2007 Cannondale Synapse Carbon Road | Selle Italia Lady Gel Flow | "Miranda"
You don't have to be great to get started, but you do have to get started to be great. -Lee J. Colan