Hi Alicia,
Wow -- fatigue is a really nonspecific symptom. Anything or nothing could be wrong!
Look for horses first, not zebras!
A few Horses:
1. Is he getting at least 8 - 9 hrs of sleep/night? High school is a whole new ball game. Classes are harder, homework takes longer and most teens give up sleep over activities. My 15 yr old DD routinely is up after 1AM finishing up assignments (all honors and AP courses). Ticks me off. On the other end of the scale, many teens stay up into the wee hours playing video or computer games, seriously affecting sleep patterns.
Also, like babies, teens are doing a lot of growing and need to sleep more to accomplish this. Fatigue can be a signal from your son's body telling him to get more rest to allow for growth hormone, growth factors (e.g. IGFs), and growth factor binding proteins (IGFBPs) production!
2. Mono (infectious mononucleosis) is a common teenage disease, even in high school. It's principle symptom can be fatigue. Other hallmarks are swollen glands and a very sore throat, although these symptoms may not appear until later, or not at all. Ask your doctor to run a mono spot (test for EBV) to rule this in or out.
3. Low level viral or bacterial infection. Viruses especially rage around most schools in the fall when everyone gets back together and starts swapping them around. Sometimes fatigue will be the only symptom. Viruses can last for 10 - 30 days or more and you can get one on top of another. I assume your son was vaccinated against Hepatitis A (which can be easily transmitted by food handling in the cafeteria), so that is probably of no concern.
4. Anemia. Pale skin and lips cand be another indicator of this, as can easy bruising and slow clotting times. Get him tested (can be done as a point of care test in the dr's office). Or, just supplement with slow release iron tablets or iron rich foods such as liver, beef, or beans for a month and see how he feels. Don't go overboard on iron, though, as an excess can cause other problems.
5. Vitamin deficiency. Even with our well-fortified foods, these can exist at a level low enough to be undetectable but high enough to cause fatigue, especially if your son is an athelete. Try adding a multivitamin into his diet for a month and see if there's an improvement.
A few Zebras (don't be freaked out by these, they are mostly rare!):
6. Crohn's disease and other IBDs often appear in the teens to 30s. Fatigue is usually accompanied by GI symptoms such as occassional diarrhea, constipation, bloating, cramping, etc.
7. Some congenital heart diseases sometimes do not become apparent until adolescence or later. Your son's check ups at this point should include a quick ECG to check for arrythmias, long QT syndrome, etc., especially if he plays sports.
8. Blood diseases, such as myeloform dysplasias and leukemias. A simple complete blood count for RBC and WBC can either rule this out or suggest the need for further testing.
9. Chronic fatigue syndrome. This is actually quite rare and more common in females than males.
10. Thyroid deficiencies. Check for TSH, T3 and T4 levels. Also, check for testosterone deficiency and other pituitary hormone levels if it seems like pubertal changes are occurring too slowly or not at all. I would think your doctor would have picked up on this though, as it would be readily apparent. Also, there is a very wide range of normal for testosterone level and puberty onset and progression.
As you can see, there are a lot of things to consider. Keep note of other symptoms. Fatigue is just too general of a term to give the doctor much information, and most doctors don't have the time to play 20 questions with a patient unless something is patently obvious.
I'm sure all will turn out well!
R.