Describe going down on him. Tell him that you are going to run your tongue from one side of his penis up to the tip and down the other side. Tell him after that you are going to swallow every inch of him and swirl your tongue on the way back up but before you do he has to be hard as a rock!
Tell him what you are about to do to him. I start by telling them how I'm going to come over, take my clothes off, take his clothes off and how badly I want to go down on him. I tell him that once I'm done doing that I want him to stick his hard dick in me.you get the point.
As a woman, you don't need to say anything. Just whisper in his ears (even if it's inaudible he will enjoy this).
Kiss his neck, ever so gently. Use your tongue, underneath his ears. Tell him that you want him, so badly.
Tell him that you love to see him hard. That you like that, and I guarantee he will be when you touch him.
I do not know of any straight man that can resist this.
Were you with him? Or on the phone?
Lick his ear. really.
Is there something he likes to do in bed that you don't let him do too often? Suggest it for a sure win. Beyond that, I think we need more details. Guys vary.