6-10 down in the 5th?

I like JKC's answer.  Think that the pressure is on him to close out the match.  In tennis, you always hear the analysts talking about "making him serve it out."  Maybe he'll feel the pressure.

In general, I agree with the advice to just play it one point at a time and not worry about the score.

But being a bit more specific, I would say serve very spinny, very short and then tactics come into play. If he is a looper then play the short game, push, push, push, push...until he tires of it and tries to loop a short low one.

I also agree that he will want to get the point faster than you and close out the game. You can use that to your advantage by putting the pressure on him and making it difficult for him to get that fast point. He will go for the point and try to get it it on a less than ideal ball.

And now going back to the first para, play it point by point, if the ball is high and can be killed, do not be defensive, kill it. If it is low and short, don't get nervous and try to get a quick point if he is out of position, play the ball on merit.

It is also a good idea to have a couple of secret odd-ball serves for situations like these to get a couple of quick surprise points. Never hurts.

This has worked for me many times.