Some people (including myself) see more results from limiting high-fat foods rather than high cholesterol foods. If you can't stand milk lower than 2%, try looking for other dairy products that are made with 2% milk, like cheese, sour cream, etc. Look for low fat icecream or frozen yogurt. What about the salad dressings you put on those salads every week? Fat can hide in some of the most unexpected places. You don't necessarily have to cut out beef altogether - just get lean cuts or extra lean ground beef. Have you tried other milk-like beverages, like soy or almond milk? I used to be a 2% only girl, but I started getting skim for cereal and other recipes and 2% for drinking. After a year or so, I was able to drink just skim milk and 2% is just too thick for me now.
You don't mention anything about your son's weight. Is his weight normal for someone his age and height? Carrying around just a few extra pounds can effect cholesterol levels too. If he is overweight, don't put him on a diet to lose unless he has surpassed his lifetime maximum (which I doubt). Any pediatrician would tell you that overweight children should grow into their weight, not lose it.