I think it's okay to answer the question because he asked your opinion.
I would say something like:
"Personally, I think a weeklong destination bachelor party sounds like a difficult and uncomfortable position to be in. But that's obviously a call you have to make yourself."
My brother asked recently what I thought about a potential living situation for him and I was able to give him some perspective he didn't have. It's possible he asked you because he has some desire to go, but already knows it isn't a good idea.