Why is my dog so food aggressive now?

My rott who is now 9 months old (male) was a friendly well behaved dog, until about 2 moths ago when he was nuetered, and he is now awful we have a 3yr old male german sheperd and a 2 yr old female rott they all get along until recently when its feeding time he sits by the dog food and starts growling he has snapped at them a few times he even growls at me when i get to close.But at snack time he will sit by the other dogs and wait patiently for his turn at his snack no growling no snarling just like the old Bruce what can i do?