So every movie ever scored has had to have adjustments of some sort. You really do not need to burden yourself with wondering if it is okay for you to ask for a change. This is YOUR movie, you have a vision for what you want, and you will be letting yourself, and your entire team down if you do not get it because you don't want to hurt anyone's feelings. So please, do talk to him, have him work over the cue, re-write entirely if necessary, it is well within you're rights, and basically, your obligation to ask him.
Now, I am a composer, not a director, but I have worked with a few, and spoken with both countless times, and this is a common question, so here are a few things to keep in mind:
1) just because the music is not what you want, doesn't mean that it is bad music, or that he is a bad composer, so be sure to say that when you are breaking the news that something needs to be redone.
2) you're right that it MAY partially be your "fault" because of a miscommunication. But as a composer, we expect this. Very rarely do you find that extremely musically talented director who can convey in musical terms exactly what he/she wants (Clint Eastwood and M. Night to name just two). And that is fine, there is no need to apologize for this whatsoever. Now if you did a bad job just generally communicating your needs for a cue in the spotting session, make sure you talk through it enough to discover where the disconnect was, and figure out how to get the two of you on the same page.
3) (and sort of tied into #2) If it turns out that he composed exactly what you asked for, and you don't like it, don't be afraid to admit that you were wrong about what you wanted. This is the most common, and most frustrating thing that as a composer we have to deal with in directors or producers is as follows - They say I want a particular cue to sound "x" (insert any descriptive, emotional quality that a scene might have. Sometimes "x" is even a specific instrument - God help us - or exact style). The composer writes precisely "x" and presents it to the director. The director is frustrated with the composer and says "this is not what I asked for at all. Not even close!". Sometimes it is just a miscommunication, but often it really is the director's instincts on the music were wrong and he/she didn't realize it until it's been scored.
4) Which brings us to #4. If you DON'T have a music background, know music theory, the intricacies of the technical limitations and variations of tone within the range on a particular instrument, then DO NOT try to communicate the musical requirements of your score in those terms. This makes it really difficult for the composer, who will not be fooled anyway, and will be extremely limited by musical specifications you may present to him, even though, without them, he might very well be able to provide you with precisely the cue you are looking for. Like I mentioned above, we're composers, we do not expect you to be or you wouldn't have hired us. So don't worry about sounding music savvy, talk in film terms.
Anyway best of luck to you in getting the cue you want. There is a fantastic book, called "On The Track" which is for film composers, but it dedicates an entire chapter to the art (not science) of talking to a director from the composer's point of view. While I'm sure there is literature out there from the flip side of that coin, I think if you can get your hands on that chapter of "On The Track" it'll give you a very unique perspective into the challenges that a composer is facing when trying to discover the music being asked of him.