Like a previous poster stated, if it's painful to the touch is prob not cancerous, just an infection/inflammation. If it is in fact a swollen lymph node, you prob have an infection/inflammation somewhere in vicinity of the swollen lymph node (your lymph nodes act as a 'guardian' and attempt to keep the infection local. a swollen lymph node is a working lymph node). Don't get stressed about worst case scenario (cancer). I've had cancer that developed near lymph nodes, and the nodes exhibited no abnormal swelling/englargment at all.
Not sure if it's been mentioned but if your sinus cavity(s) and teeth all feel fine, and you have no other pain in the head/neck area other than what you mentioned, it may be a sign of mumps. Google mumps for more info.
As far as rotting teeth in your mouth, a rank smell would def be emitting from your mouth and there should be some inflammation of the gums around problem tooth/teeth. Maybe ask a 'loved one' to take a whiff for decaying matter. lol, but seriously.
Good luck with this dude. I'll keep my fingers crossed that it's nothing serious; not that crossing my fingers will do anything, but you know what I'm sayin'.
