I’m worried about my doggo

I think he might have been abused by a previous owner (he was found abandoned with another dog who resembles him and might be his sister or mom, seemingly well cared for but unneutered), he flinches when you approach him in certain ways, and got scared when I picked up my selfie stick, and startled when I stood up off the sofa.  He definitely seems a little anxious (which like first night in a new home, it’s normal to be anxious).  He’s suuuper obedient (except for “stay” at the top of the stairs, he just seems to really like running down stairs).   He’s a total cuddle bug and very clingy.