Find a quiet time when there is nothing to distract either of you.
Sit him down and just tell him straight out.
There is no softening the news, but make sure you have all the information at hand, he will probably want all the details.
Have a stiff drink nearby for him, and let him talk till there is no talk left.
Be there to listen, console and support, you can't stop the grief, but you can help by letting him grieve.