Dear R.,
You've gotten some great advice here. I just wanted to second one recommendation and respectfully disagree with two others. First, I really agree with Leona -- five is awfully young for a high cholesterol diagnosis. I'd have that re-checked. A child that young needs some "good fats" in his diet (olive oil, avocado, etc.) for optimal brain development -- a "false positive" on cholesterol could actually be a cause for concern.
Second, the two things suggested below that might be problematic are soy milk and tuna. Most soy milk is made from genetically modified organisms, which have been shown to have a deleterious effect on the hormonal development of little boys. Some organic brands of soy milk are okay, but you might be safer with rice, oat, or almond milk. And tuna is VERY high in mercury. Fish is wonderful, but nutritionists recommend that everybody, especially children, consume the fish that are lower on the fish food chain -- tilapia, mackerel, herring, etc.
All the other suggestions (oatmeal, etc.) look wonderful.
Best of luck,
Mira