L.T.
I'm assuming you have already talked to your son's pediatrician since you know he has high cholesterol. Did he happen to recommend a dietitian? Something like this requires healthy eating habits and not a short term diet.
Fresh foods such as fruits and veggies are a good choice. Buy meat that is lean and cut off any visible fat before preparing it - skinless chicken is good. Cook with no or as little butter/oil as possible. Limit sweets and processed foods. Serve low fat milk such as 1% or skim. Whole grains are supposed to help lower cholesterol, so Cheerios and oatmeal are good. Switch to whole grain bread, crackers and pasta. Avoid frying foods.
Check out this website for some recipe ideas and cooking methods. If you like what you see you can sign up for their newsletter. http://lowfatcooking.about.com/
Hope this helps.