I'm going to second nycindie. Rules about who can and can't talk to someone outside the primary relationship, especially if the outside person was interested in both of you, seem silly.
Additionally, if I may be so bold, his question to you about why you would feel uncomfortable with him going on a Hawaii trip with someone the two of you met together seems at best misguided and at worst manipulative. It sounds like the two of you met Dennis recently, and given that your communications with him were limited, you really don't know that much about him. The question is why would you NOT feel uncomfortable with that situation?
Personally, this whole situation sounds kind of fishy, and I think you have every right to feel uncomfortable. I can't really suggest anything except to keep doing what you're doing: being open with him about your feelings, even if that is scary sometimes. Hopefully if you continue to do a good job of that (it sounds like you are), he will follow suit.