Being tied up, it may prove difficult for him to satisfy you. This is his problem, though, not yours. Tell him he needs to try harder and slap his cock and balls HARD (remember, they'll be fine, and he has a safeword that you're dying to hear...). Put your weight on his face, bury it in your cunt. Continue abusing his balls: squeeze them, shake them, slap them. Let him breathe periodically, just long enough to tell him that you think he can do better.
Keep it up until you think he's had enough, then keep going anyways until YOU'RE satisfied. This is about your desires, not his.