So sorry to hear about your MIL. My best to you and the family.
About your son, you probably don't really have to worry about the lymph node on his neck. You have to worry about the one on top of his collarbone. THAT'S one you don't want to see.
My son had one that made him look like Frankenstein. That was awful. He was very sick with bloody poop and we were waiting for the stool sample results to come in. I took him back to the doctor when the node got big (before the stool sample results came in) and he told me that he was fighting an infection, and we just needed to see the results of the stool sample to know which infection it was to be sure of how to treat it. To be safe, and because I was back in the office about to have a proverbial "heart attack", he took blood this time to see what it showed.
It turned out that my son had salmonella. His body was fighting that infection hard. This is when I found out that the lymph nodes up on top of the collarbone are the bad ones to get swollen (it can mean leukemia). So his body was fighting this so much that his neck lymph node was huge. (Thank goodness his collarbone lymph nodes weren't affected.)
Anyway, I hope that they are trying to pinpoint what your son's infection is. It takes a while, but the lymph node goes back to normal size. If it doesn't, then it's something to worry about.
That's our experience with a big node on the side of the neck.
Hope he's better really soon!
Dawn