He will be ok. Normal. How old is your dog? Nuetering him won't make him fat if you control his diet and also give regular excercise.

Your dog won't have much behaviour change... you will see changes in the first few weeks but later he will go back to his old self. Most of my dogs are spayed and nuetered, no problems and no weight issues too.

He will also socialise better with other dogs if he was dominant before nuetering.