For example, if he starts talking about his stories again, redirect the conversation by starting a new topic.
Yes, in this scenario, you could first try to shift the topic of conversation naturally by asking him questions about other aspects of his life. Or even other aspects about the girls and the parties he's talking about.
Even if other aspects of his life don't interest you in and of themselves, you could approach those topics vicariously -- taking an interest because you care(?) about him and for the sake of your relationship and keeping connected with him.
That's something you might have to do every time you talk with him. Shift the topic of conversation every time he mentions it. Over time, as you keep steering the topic of conversation away from what you don't want to hear, your brother would (hopefully? theoretically?) naturally start avoiding it.
if you're unable to sway the conversation, you might need to just tell him outright you don't want to hear about that anymore.
A very blunt approach like this could do the job too.
I would qualify this based on the current strength of your relationship. For instance, if this is
all he ever talks about with you, then it's possible he could receive it as if you're telling him you don't want to hear from him at all anymore.
But if you have at least a moderate-to-strong (well-rounded?) relationship with him, then maybe a blunt, direct statement (repeated as necessary) is all you need.
What's the key to setting boundaries? I know what I want is valid but that isn't enough. I can't even get the words out even though they are on the tip of my tongue.
In establishing boundaries, you want to be careful with your phrasing to not only avoid actively judging the other person but also minimize the risk of the other person even
perceiving a judgement. (Even if you intend no judgement, your
intention alone doesn't negate this risk.)
Use "I" statements (eg.
"I feel __________ when __________"). Avoid "you" statements as you can (eg.
"You make me __________.")
So, you could say something like:
"When I hear your stories about girls and parties and excessive drinking, it makes me feel __________."
Fill in the blank with whatever it makes you feel, and then tell him what you need him to do differently. Reassure him that you value your relationship with him and that you want to hear about what's going on in his life but that there are just some topics that are __________ for you (fill in the blank with whatever you feel). What he could do differently, you could say something like,
"It would help me if you avoided those topics when we're talking."
You could end off by saying something like,
"If those topics come up, I'll probably change the subject, but that's not a 'you' issue; that's a 'me' issue." (Then you could reinforce how you want to stay connected with him, if that fits for the moment.)
