Some good advice in this thread so far.

I've had this happen myself - probably about 1/3 to 1/2 the time - and in my case most of the problem is a certain position. We do it because it works for my wife - she generally manages to orgasm repeatedly as long as I can hold out. Unfortunately it generally takes about 2-3 minutes of this position to completely flatline me, so it is a bit of a chess match.

Of course, if it is a regular occurrence it would be worth discussing with a doctor he trusts. Just be understanding and make sure he knows there is no pressure.