If you feel uncomfortable, try turning the lights off, getting under the covers, and just whisper to him with roaming hands, rather than fumbling for words under a 60 watt bulb with him looking on expectantly. Or even try surprising him with a sexy phonecall while you're at home in bed and he can't do anything about it. This way you'll be in control the first time.
Lying on your back changes the pitch of your voice, as does projecting your voice from your navel. This may sound crazy but try it.... Say talking as if your voice is coming from your bellybutton. Instantly more sexy. Don't talk too fast, and make it a little breathy. Leave out the pornstar screaming and filthy language UNLESS that's what he likes. Breathy moans, gasps and whispers are generally sexier.
Keep on the same level of language as him, so for example if he is saying "make love" and "breasts" don't start in with the c word, and vice versa. Decide on the words you're comfortable with using and don't ever be tempted to go into "throbbing love pole" territory unless you want to ruin the mood and end up in a fit of giggles instead!
As for WHAT to say, is there any aspect of what he says that turns you on? Men like to hear their own fantasies being echoed, so say if he's talking about an illicit encounter with you where you meet as strangers in a sauna and you get wandering hands in the steam, try to get in on it as he's describing it and embellish it but carry on along his theme. Make it hotter and wetter and him more irresistable, and consider all your senses (and the fact you could get caught/someone may be watching and/or wanting to join in) etc etc...let your imagination run away with you.
Don't forget to let him know how turned on you are/how much you want him/he's driving you crazy/don't stop. You get the picture. Throw in the odd command too, and make the most of your position of power. You may find when you loosen up you begin to enjoy getting him in a state!
And as someone just said, your own fantasies are a great place to start as long as you know/suspect he would find them a turnon and not odd/scary (best leave the ones involving your sexy personal trainer/massive bullwhips at home and start off fairly conservatively) Just have a think about it next time you "indulge yourself" and try whispering the fantasy outloud to see how it sounds. Just remember to turn the two-way baby monitor off first.
Chances are he'd love to hear what goes through your mind, and love adding his own input to it. Warning: This will usually involve him, his fantastic phallus, and half the members of the pussycat dolls, ruining your milkmaid in a hayloft fantasy forever. Perhaps keep your best ones to yourself!
And no, for anyone who's wondering, I don't talk up a storm to my dp every night (I let him have the odd wednesday night off) I just used to have a summer job on a chatline so had plenty of (boring, repetitive, unsatisfying) experience with entertaining men solely with the powers of my imagination and precious little of theirs.