This is a longstanding problem with "Ford" that started when he could no longer go outside. My suggestion is to get a harness, let him get used to it in the apt. When you decide he's accepting of the harness, make sure it's snug on him, and take him outside. It's not that he doesn't know how to use the litter box, he does occasionally, but he wants to go outside. Let him guide you where he wants to walk. Since he's high strung & skitterish, take a towel with you to wrap him up if he throws a tantrum if you're trying to get him back to apt. Perhaps with 2 short walks a day, this will relieve his frustration, and hopefully he will be content to use the litter box. At times when you're away for any length of time, confine him to a medium dog-size crate with litter box, or the bathroom. Worth a try.