You went to QuickCare and the doctor told you it's likely caused by a virus. Lymphoma is a rare disease. I would not make the leap from a virus to lymphoma. Relax. A swollen lymph node is rarely serious. Multiple swollen nodes with a fever and a rash would be a reason for concern but one lymph node that has been seen by a doctor is not a reason to panic. Trust the doctor you've seen. If you don't trust him/her then call your son's pediatrician. But above all do not freak out about it. That's turning something minor into a major incident which is more dangerous to your health than the lymph node is to your son's health. And,,,,your anxiety will influence your son's sense of well being.
BTW I have a swollen lymph node in my neck that has been there for 20 years or more. Not a problem. Doctor checks it with each physical exam, looking to see if it's grown. It's not and so it's harmless.