I wouldn’t worry about it. My guess is that he’s done getting taller. Most puppies stop growing up long before they finish filling out. From what I’ve heard, a lot of dogs are done getting taller by 8-9 months. 25” is plenty tall. Don’t worry.
Male GSD is 'officially' his between 24-26" at the withers. My dog is 25,5" and he looks short if there's another big gsds. But compared to other dogs in general... He's quite big. So 25" is not a problem at all and his youth looks might just be his looks. He IS still 11 months