I would hold off on neutering him until he is between 18-24 months and more physically mature.
I agree with LoveEcho about feeding a different kibble, I have had success with Taste of the Wild, Blue Buffalo Blue Wilderness, Solid Gold and Wellness Core. Although Wellness Core is my favorite, Taste of the Wild is more affordable.
I don't think that you are putting the weight on too fast.
Sinister "The Prince of Darkness" 4 yr old black male GSD 3.11.09
Draco "Dakota's Unbreakable Vow" 5.5 mth old male Dalmatian 6.30.13
Cats: Chaos, Mayhem, Monster, Wicked