1. Arouse his curiosity
If your guy's really shy, don't freak him out by asking outright. Instead, start by telling him that you're enjoying your romps, but you'd really like to bring the sex up to an even steamier level. Pause for a little suspense: He'll be all ears by now. Tell him how by sharing your erotic fantasies, you can seek new ways to please him and vice versa, and by adding that "it'll bring our relationship even closer."
2. Set your boundaries and expectations
Establish boundaries that you're both comfortable with. For example, set a rule of revealing only fantasies that involve the both of you (and not a third party), so he won't accidentally blurt out something totally off-limits -- like a secret fantasy of your best friend.
3. Share your fantasies too
If you want your guy to share his fantasies, then show by example and start sharing yours. Tell him what turns you on, describing in titillating details. Remind him that just because you share a fantasy, it doesn't mean you have to act it out in real life -- it'll help put him at ease and get him to open up.
4. Discuss during foreplay
It's no surprise that foreplay's the best time to share your fantasies: You're both in the mood and your inhibitions are down. Even better, if you have an inkling of what turns him on, try describing what you're going to do to him (or even acting it out) and gauge his reactions.
5. Ask questions
If he finds it awkward to spell out his inner fantasies, try asking him simple "yes" or "no" questions, like "Have you ever fantasized about me being in charge during sex?". If he says yes, ask him to elaborate.
6. Offer him options
Give him a list of naughty role-playing options to choose from, like "Let's try something fun today! Since you've been a darling lately, I'll let you make the choice. Would you like me to (A) dress up as a schoolgirl or (B) play your secretary at work or (C) try a little S&M?" Since you're the one who proposed the ideas, he knows he'll be able to choose whatever he prefers from the menu without you freaking out.
7. Do up a not-list
Rather than revealing your steamy imaginings, it may feel safer for both of you to share what you're not into. Write down a list of things that you wouldn't want to explore or which turn you off, then exchange this list. You'll be able to narrow down the possibilities and take it from there.