HiCustomer Thank you for asking your question on JustAnswer. By the way, it would help us to know:
-What was his childhood like? Did he get enough attention for his parents?
-Did you consider counseling?
Thank you again for trusting us with your problem. Please reply as soon as possible so that we can finish answering your question.
Some character traits are inherited but it can also be a learned behavior from trying to get attention because of having older siblings. It may be a compulsive behavior that he may not be able to control. It's important that he go to counseling for this alone so that he won't find himself having to fight for the counselor's attention. He had to deal with alot as a child and this could be in part why he talks too much it could be a nervous condition also but his Doctor will have to diagnose it. I would both ask him to start counseling and visit his Doctor to see why he does this the Doctor could possibly give him medicine for it, there is such a thing as Adult ADD but I'm not a health expert so I don't know what symptoms come with that. It's important to get him diagnosed.
Would he at least go to the Doctor and ask him about it and maybe he can diagnose it for him. He has to do something if it is that embarrassing for you, you want to find out why he does this. If it is something he can correct he should want to do whatever it takes to get the help and enhance your relationship and then you will be able to socialize with other people without interrupting them or talking over them.