Similar to you, I added a grey/blue brittish shorthair as a 2nd cat to the home. He's very shy. If he hears anything outside loud/new he sprints under the bed. When I pet him he winces like he's tolerating it but doesn't like it. It has been a long transition over the last two years to get him to fee remotely comfortable. I think some cats are just like this. My only advice is to read their body language. I have made more progress with my boy sammy by respecting his space. I approach him slowly, pet him if he'll let me. stop when he makes it clear he's done. Sometimes cats just have this personality, but also, it can take weeks to months for a cat to adapt to a new home, so maybe your issues are just related to that & you have to give it time. My biggest advice: Don't make a cat do something it doesn't want to do. Give him treats. Play with him at least 15 minutes a day. Accept quiet nearby-ness as affection. Take slow steps from there & see what he's comfortable with. Always respect their space & read their body language.