From the emotional angle: you'll relax naturally as the two of you settle into your own rythm. It takes time, that's all. The next time you play, do it without the expectation of having sex. In fact, have a session where you can go as far as you want EXCEPT for actual intercourse. Knowing that it's not an issue takes the pressure off, and knowing that you can't do something instead of having to follow through with the fucking will make it all that much more desireable.

From the physical side of thigs: get a vibrator that is smaller than your Dom. Have him use that, and have him use his fingers until things relax and loosen up a bit. Baby steps.