D.B.
Well, the good news is that nothing awful has shown up on the tests so far. Maybe you'll get an answer from the blood work.
While a swollen lymph node is usually the sign of the body fighting off an infection (a good thing, actually - the immune system at work), it's only going to respond to antibiotics if the infection is bacterial. If it's viral, it often doesn't show up on any test at all, and it just has to run its course. He might not have had other symptoms enough for you (or anyone to notice) - I had a swollen lymph node in my neck for a few weeks, with no other symptoms, but who knows if there was something lurking "beneath the surface"? We just waited it out and my doctor said that, if there was no change in a few weeks, we'd take the next step. That never happened.
I totally understand that this is scary for you, but it could well be the body's natural response to something minor. I hope he will be fine and that you will get some peace soon!