We had an icky virus too this year. In feeding my kids during these times, I think about how I feel when I'm sick. I tend to eat what I feel like eating, because almost nothing else sounds the least appetizing. Why should children be any different? I don't really eat only crackers and clear fluids when sick...
If he wants mac n cheese, why not boil the macaroni in chicken broth (for some added nutrition). I liked the grated cheese idea someone else had or leave it plain. My son was nearly three when he was sick and I explained at every meal "No cheese or yogurt until your poop is back to normal." One day he came running out from the bathroom and called me back to check it out. All normal, time for dairy again. Also, you could try to give him lactose free milk. My son is lactose intolerant. When people esp. children have diarrhea, they can become lactose intolerant until the good bacteria have a chance to recolonize the digestive system. This causes more diarrhea (and cramps if severe enough).