It's a lot to ask for a fifteen-year-old cat to adjust. Likely at best they will get to tolerate each other. I brought in a young, adult rescue cat last fall. My fifteen-year-old cat and this cat still don't really like each other. The rest of my cats are younger and have adjusted. I would just let the fifteen-year-old go where he feels comfortable. They need to sleep a lot at that age, so he likely is just looking for somewhere quiet to hang out. It sounds like they are not doing too badly, considering the circumstances.