I'm under the impression that genetics can also play a role. My cholesterol has always been low as is my blood pressure (which while preggo with Suki and living off of chipotle made my OB's eyeballs spin.) I had a friend in college who worked out for an hour a day, ate pretty healthy and still had ****** cholesterol.
The paper work I got though had 180 as the upper range of good, so 180+ was time to see the doctor.
As for explaining it, I would ask myself if it is necessary. A five year old may be intellectually precocious but they are not emotionally developed enough to cope with Mommys health crisis. It might just be one of those things to either not tell him or hold off till he's in his preteens. However, knowing that you have issues with cholesterol may help out when you go for checkups, especially if it is something that runs in the family.
Browsing the web, the only example they have is how to explain it to a 10 year old.
http://thedietsolution-program.com/blog/explaining-cholesterol-to-a-10-year-old/
Just weigh the consequences and benefits:
1. Nomatter how well you explain it, will it upset him?
2. WIll it cause him anxiety due to worrying about Mommy?
3. How will it help him?
thats just my