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.