I have 3 kids boy 9, girl 11, boy13. We told them all together, but in very general terms. Basically, Daddy's heart needed fixing and I was going to be in the hosptial a few days. This was fine for the 9 year old, and he thought it was cool because my surgery was scheduled on his birthday. For the 2 older ones, showed them some pictures and try to explain about the valves and how mine was "broken". One thing for sure - do not underestimate how much they can understand and comprehend. My wife and I were both very positive about it all and try to make them as comfortable with it as possible. But the older 2 completely understood the seriousness and although they never expressed it me, were frightened. I found this out later through emails and conversations they had with their friends.
It was kind of interesting when I came home and how they each reacted. I showed them my chest and the oldest one just looked away, my daughter cried, and my youngest thought it was really cool and made a point of showing all his friends when they came over.
Basically, just be open about it. All parents know just how much there kids can handle and I am sure you will do great with your son.