If you have a close bond you shouldn't worry too much. The best thing to do is ask him just out of the blue what he thinks of BDSM,and ask him would he be more Dominant or Submissive.You also need to know that because if his a sub well then u might have a problem unless he can do both and switch.
If you get a positive answer out of him then continue it with umm how u fantisied about a man domminating you.Then see his reaction.
I don't think he'll freak out most guys love to hear what their girls fantizise about...
Good luck!