I'm an unfortunate expert on lymph nodes having been diagnosed with lymphoma (specifically Hodgkins - cancer) after the birth of my daughter almost 2 years ago.
The fact that they're rubbery and movable is a really good sign. 95% of all lymph node enlargements are to help the body process infection - chances are, your son is fine, and his nodes are needing time to rebound. I fell in the 5%, and I really had to press the issue. The location of mine was the real concern (collarbone area). If they're tender, that's also a good sign as cancer cells have no nerve endings and won't cause pain in the lymph nodes as they enlarge.
In my case, I found a swollen supraclavicular node (collarbone) after nursing my daughter one night. I went to my OB the next day who "wasn't concerned" because they were small, not really hard, and movable. I went to my Family Practitioner the next day who ordered an ultrasound and ENT consultation.
In the meantime, I returned to work from maternity leave, had contact with literally a few hundred physicians as I was in pharmaceutical sales at the time, and no one really thought it was anything other than post-partum hormones.
The radiologist determined "benign adenopathy", and it took 3 pathologists to confirm my Hodgkins diagnosis.
If your mommy instincts tell you to press, DO IT! The worst outcome if he's OK is that you have peace of mind. Do not get a fine needle biopsy. While it's the lesser invasive type of biopsy, it can often draw inconclusive results. A full biopsy is really going to be more conclusive.
If your instincts are telling you to be concerned, start taking action now and don't wait. I was fortunate to be diagnosed at Stage II - early stage. And, after 5 months of chemo, I've been fortunate to be cancer free for over 18 months.
Lymphomas are common in children - especially Hodgkins with peak ages of diagnosis in the teens to the mid-30's.
Feel free to contact me if you have any questions or need direction.
Chances are, he's fine!
Good luck!