My ten-year-old son has begun to have horrible headaches. Sometimes they last for days, and they have begun to interfere with school and sports.?

?migraine. See the doctor for a thorough evaluation to rule out other conditions and start treatments. Keep a log of his headaches and any triggers.
Needs workup. It can be as simple as needing eyeglasses -- or as critical as a still-curable brain or pituitary tumor. This requires a full workup by a competent physician. Don't delay.