Can i ask my boyfriend (best man) details about his brothers bachelor party?

You need to search yourself and decide if you can deal with not knowing. If you can, then you should drop it. If you feel like you need to know, then tell your boyfriend that you absolutely have to know what happened. If he doesn't tell you, then maybe something happened that he doesn't want you to know, because he thinks you'll be mad. You need to talk to him.