Hiya.
In complete contradiction to doggydays, I would suggest that unless you are breeding from a dog there is very little reason why he shouldn't be castrated.
I was told when we had our BC boy done at about 14 months that it would take around 6 weeks for hormones to subside. He stopped marking indoors, and was calmer, and less wary of other males. I understand it differs from dog to dog, and that in some cases it's claimed to have no effect on behaviour, but that's our experience with Jack.
If your dog is "complete", he can be more likely to try and escape to follow a female's scent, or wander off when you're walking off the lead. It can also lead to unpleasant and expensive accidents with unspayed bitches! We decided to get Jack done partly in the hope that he would settle, and partly for the above reason. Also, it prevents any cancers "down below". It seems a healthier idea all round, really.
Hope your pup is all recovered-let us know how it goes!
Lisa x