Boston The Wild Cat

Introducing Boston, an urban multi-generational wild cat humanely trapped and brought to me. He is around 4-5yrs old at a guess and I need to get him to the vet for desexing. I have had him for around 3wks at this stage and he won't use the cat cage inside his crate, instead preferring to sit in the cardboard box that should house his litter tray. He kept sitting in the litter tray so I moved it so he could have the box. Boston won't be suitable for rehoming but I'm hoping he will stay on my property where I can feed him.