There is no easy way, but telling him calmly, and in the most caring way possible would be my way of doing it. As for the boys reaction? who knows, we all react differently, but stand by for whatever way he reacts, and be a solid support for him.
tell him gently and don't worry how he will act he is going to be destroyed.<br />
Sit him down and explain his mother is gone, she will be watching him forever but he will not see her. He will feel her, she still loves him, and misses him deeply. He may need a professional if he can't cope
Be truthful and explain that this is one of lifes realities. Make sure you let him ask questions and answer him honestly. Also make sure he knows he has someone to come to when he needs. I had to do this years ago with my 7 year old daughter. Make sure you allow him to grieve in his own way.
You have to tell him.... He is still young, he will be upset. Possibly he will recover quickly at his age.