Nothing wakes him. He has trouble getting to sleep, but when he sleeps, he's out for 12 hours or more. He is in excellent health, but he's missed a lot of school during the last two years because he can't get up. Your son doesn't sound atypical of many teens. If he's in good health otherwise, his problem may be his sleeping patterns.
Teens tend to sleep irregular hours, staying up late and sleeping in until noon.
This can become a habit so that getting up early in the morning becomes a major feat. Also at this age, teens tend to sleep more deeply in stages three and four of the sleep cycle, so they sleep through alarms, thunderstorms, etc. Your son should first try to have a somewhat consistent bedtime and also start waking a half-hour earlier in the morning the first week, then one hour the second week, etc. This probably will not be easy. If he tries creating a scheduled sleep time and still has problems, consult his primary care doctor and find out if he needs to be evaluated in a sleep lab.