Try to give him a break for a while... seems funny how you hear about women not wanting sex that much or denying a guy sex --- that's normal... but for a guy to not have the ability or desire, it becomes a state of emergency.
If this is psychological, then making less of an issue of this can only help. Men pride themselves on their sexual abilities, it can be discouraging when they seemingly lose that ability... and downright crippling if the woman they love brings attention to it, demands an explanation, demands a solution, and demands that he 'get help.' Even if this is more of a physical problem... he'll need time to come to terms with this on his own, because this is a very personal problem and may even be devastating to him -- so sensitivity is needed on your part.
Psychological or physical problem... he needs time to either relax enough to 'get it back' or have time enough to come to terms with it and look for a solution because he wants to.
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen