None of my males changed their body composition after being neutered. I adopted a four year old that was neutered at that time and he went on to be a rippling muscled boxer hunk for the rest of his days.
I currently have two males neutered at age six months and neither seemed to be changing. In fact, my 2 year old is like any young male, tons more metabolism than he can eat for! He can eat and eat and never put on an ounce of fat.
All my friends have neutered males and even the ones over five years old all look terrific! One is often mistaken for a competing show dog at shows until people see that he was neutered (at 6 months!)
I just assumed: Quality food in, quality body.
Character??? I don't know. But having owned three neutered males and fostering many (that were neutered while in my care) I will always have neutered males in my house. They are FABULOUS furkids. Mine have the most wonderful, loving pesonalities but with no negative male behaviors. I think that wonderful male boxer personality just keeps shining. I have no worries having my big boys watching over and loving me. They are love bugs, alert to the environment around us, and very much momma's boys.
Fit, trim, and neutered - Tulley and Desi
owned by me- Ashlie