the vet complained that he tried to bite him when he was being examined after his operation, he also seems to be showing alot more dominance.
he has also started growling if you go near his food whilst eating, something he has never done.
The effects of a castration will NOT show up as fast as this. It takes weeks, if not months for the hormones to settle down so what's going on isn't down to the castration, other than he's probably sore and hurting after the surgery. Your vet should have been capable of dealing with a dog who is showing aggression without telling you he tried to bite him imvho!! What he may have seen was fear biting, which is down to him being nervous of his situation. He may be picking up some vibes from you (after being told about the bite thing). This does need to be stopped (the defending food thing) however and I'd suggest once he's properly healed (last week is well early for him to be completely over the effects of the actual surgery) you get him to some training classes.