How long until my dog's behavior changes after neutering surgery?

Initially probably you won't see much changes, at least for the first few months. Slowly attitude may change towards less markings and other male-sexually related activities (aggression, escaping looking for females and so on). Most of the times these changes influence positively their life in my opinion.

Withering of male dogs is advised between 4 and 6 months of age. Here is wonderful resource to explain the many benefits; https://www.petcoach.co/article/5-great-reasons-to-spay-or-neuter-your-pet/ The only behaviors that will be affected by castration are those that are under the influence of male hormones. So for example, a dog’s temperament, training, and personality, are related to genetics and upbringing, not hormones. Neutering does not “calm” an excitable dog, and unless a castrated male dog is overfed or underexercised, there is no reason for it gain weight or become inactive. The undesired traits which are related to male hormones such as sexual behaviours, aggression, and urine marking will change over the course of a few days to a few months. I hope that is helpful information!

It depends on what you mean by "calm him down." Neutering will generally make male dogs less apt to roam in search of females, and may also decrease mounting and urine marking behavior - but otherwise would not be expected to have any significant effects on personality or energy levels.

Castration will have the following benefits: No more icky discharge from the penis or prepuce, no incentive to roam and look for females, if the dog shows some aggression, castration will usually lessen aggressive behavior. A lot of entire males are constantly upset and excited if they smell females in heat ( and the smell can travel for a mile and more), No chance of making babies. Here is more info by the ASPCA: http://www.aspca.org/pet-care/general-pet-care/spayneuter-your-pet

Usually after neutering the hormones decrease gradually with most gone in a few weeks, up to six weeks. Once Rocky is neutered you may want to have a trial foster to make sure the dogs will adjust to each other and not fight. Some older dogs still act more male even after they have been neutered and I don’t want you to adopt a dog that doesn’t get along with Bosco. Thanks for using Petco Pet Education Center, formerly Petcoach to help you get answers to your veterinary questions.

This really depends on what he is doing, and how long ago he was neutered. If the surgery was today, then he will likely be feeling strange and not himself for the whole evening. Depending on what kind of anesthetic drugs and pain medications he was given, he could be really whiny, needy, or just laying around. If he is really agitated, or trying to lick constantly, or seems really painful, then he may not have been given adequate pain medication and I would call your after-hours vet right away for further help. He may not be interested in food or water, and this can be normal, just for tonight. Tomorrow he should be eating and acting normally. If he is laying around and won't get up even when you stimulate him, this may be an emergency and I would call a vet right away, as this is certainly not normal. With my neuters, I always send them home the same day, fully awake, and with extra pain medication to go home with. Hope that helps!

5 years is not too old to be neutered. Some behaviors are induced by hormones , and they can also turn into habits independent from hormonal activity, Will he stop roaming? Possibly, but maybe not. Will he become less aggressive: most likely. If you want to know whether neutering helps or changes his behavior, you can have the vet give him an injection that suppresses testosterone (chemical castration). If you are satisfied that this works, you should have him castrated. If it does not change his behavior then castration will only serve to avoid uncontrolled procreation.