Should I take him on the cruise?

I just found this wonderful site and am thankful that there is such a thing. I have a question for those who may be able to help with their experience and advice.
My nephew is 15. From what I have observed from his behavior and the few articles that I have just started to read, he fits the description for IED very very much.
I have bipolar disorder but this does not look like what he is experiencing at all. Our family background has a history of mental illness in it.
I have observed his explosions for several years and they are only getting worse.
He has started therapy recently but has not been diagnosed with this disorder as yet.
This weekend I observed some behavior and he told me some things that really disturbed and concerned me for his safety and the safety of others.
He, my partner, and I are supposed to go on a week long cruise to Alaska in July but I am concerned about taking him at this point since he is not being treated for this disorder.
I am concerned about him, others, myself and what to do if he becomes uncontrollable.
Thanks for your time