Have you tried taking him to obedience classes? That is the most effective solution I have aver seen for behavioral issues. Here are some past discussions about barking, hopefully they will have some useful information for you! Good luck!
I thought this one especially relevant - http://www.mycorgi.com/forum/topics/will-neutering-help-with-my
Barking is definitely NOT aggression. I'm assuming he's pretty young if he's just getting neutered? A lot of corgis tend to "find their voice" as they get older; puppies that barely made a peep may turn out to be very vocal adults. It sounds to me like he started barking out of boredom from being confined and now he thinks that it is an acceptable behavior. I'd follow Melissa's training advice and remember to be very patient, it won't happen over night. If you have a trainer they should be able to help you out quite a bit as well.