Stenosis means an abnormal narrowing of some opening, canal, or vessel. In this case, we are talking about some stenosis affecting either the cervical spinal cord (central) or nerve roots coming off of it. Congenital stenosis is typically central. It is usually asymptomatic and does not require restriction from any sports. It is not associated with a measurable increase in risk for catastrophic injury. Central stenosis is only a concern if there is some other superimposed risk factor suggesting permanent cord damage may occur: multiple episodes of transient quadriparesis, for example. The number of cases is still very small, but the downside is so terrible that players may be restricted for life by some doctors.
Discs can bulge, tear, and squirt their inner gel-like contents in a number of shapes and sizes. This can be superimposed on underlying bony stenosis or may be the only factor. Discs can cause cord (central) and/or nerve root compression. Discs tend to heal over time, and extruded material which compresses nerves often resorbs well enough to resolve all symptoms and risks.
Acquired degenerative spondylotic stenosis is quite rare in teenagers.
If his problem were isolated low-risk congenital stenosis, none of this drama would be going on. If his problem is high-risk congenital stenosis (e.g., with multiple quadriparetic events), the guy holding him out is unlikely to change his mind later (though he might get a second opinion from a less risk-averse physician). If his problem is largely related to a disc injury, it may get better with time, allowing return to play. If it does not, he may have a good result with disc material removal and fusion of the bones on either side of it to each other, again potentially allowing return to play.
The information dribbling out leaves something to be desired, but it currently sounds like some disc is involved. The timeline for giving non-operative care a chance will apparently run long enough that any surgery is likely to keep him off the field for 2019. Unless an official release offers more specificity, we will be left to do a lot of guessing. Every time "disc" gets mentioned, though, it is a positive sign, because that is a problem which can often be fixed and careers resumed.