What could cause a healthy seventeen year old boy to fall into a coma without any injury?

Several causes. Among the possible causes - intracranial hemorrhage, outcome of status epilepticus, underlying metabolic disorder that was asymptomatic but was triggered. In these situations one muct also consider toxins or effect of drugs.
A Few Reasons. There could be several possibilities, including toxicity or an infectious disease process.
See below. There are many possibilities for this problem, most concerning amongst them would be infection. Other possibilities include bleeding and chemical use. A neurologist would be able to further help with this problem. I am sorry that I cannot be more helpful here.