Why does my neutered dog only hump one specific male dog at friend's?

This is not a sexual behavior, but likely a response to stress, anxiety or to assert his dominance. Correct the behavior immediately when you see it. Remove him from the dog with a stern "No" and offer a toy or task to distract him. Praise and reward him when he complies. If he persists, separate him until he settles down. Be firm and be consistent.

Has anything changed in the household? New people? New pets? Babies who started crawling/walking? Construction? Guests? Change of furniture? Changed schedules of routine? Etc.. Sometimes when there is some type of change it can strike up this behavior because they are anxious and trying to figure out what to do with themselves. They can also do it if they are trying to portray dominance. You can tell him "no" and take his toy away. If there is a particular toy that he likes to hump, I would get rid of it. You can also give him a calming chew like composure and a calming collar like adaptil. I hope this helps!

Humping is not only a sexual behavior, it can also be a dominance behavior, a stress response, etc. Any new changes lately - new pet in the home, any major changes in his routine or in the house? I suggest you refer to this article for more information. Hope this helps. ( https://www.aspca.org/pet-care/dog-care/common-dog-behavior-issues/mounting-and-masturbation )

This could be a dominance behavior or may be due to an abnormal production of hormones such as cushings disease. I recommend a full vet check and bloods especially if there are any additional symptoms such as altered appetite and or coat changes

Dogs can still hump things even after being neutered because they have learned that it feels good. When you see him getting ready to hump something, immediate distract him with another toy, or ask him to perform a basis command you have taught him. That will get his attention away from humping, and gives him an alternative behavior to perform.

It is likely your dog is doing this behavior to your brother's dog because they are around each other long enough for your dog to have the opportunity and maybe he feels comfortable with your brother's dog if they are around eachother frequently and is not fearful of your brother's dog's reaction or he really enjoys seeing your brother's dog which makes him extra excited. You can try to distract him with a game or toy before the behavior occurs to discourage it if this bothers you. Having him neutered may also decrease this behavior. You can read more about this issue at https://www.aspca.org/pet-care/dog-care/common-dog-behavior-issues/mounting-and-masturbation

He is coming into puberty with lots of hormones. Most of this has to do with him trying to be in charge and leader of the pack. He does it to you because he does not think you are the boss, to where your boyfriend is so he is submissive. The best thing to do is get him neutered. You also need to be firm and enforce the rules to show him you are in charge also.