Pure speculation on my part, but if I had to take a guess from my living room couch, I would say his meniscus. The fact that he can run/play and seem fine somewhat, then jump and land wrong on it and hurt it, makes me thing something like a meniscal tear. If it is indeed the meniscus, they will probably scope it on Sunday, Dec 2. Clean it up and let him heal up in prep for the bowl game.
If it is a sprain, stay off of it and rest it, and get ready for the bowl game.