It's very possible for tests to come back and NOT show anything serious. If you or he believe something is really wrong, keep pressing.
Yesterday was the 2 year anniversary of my cancer diagnosis. My ONLY symptom was a small swollen lymph node near my collarbone. I had no fever, no nausea. Yes, I was tired and was losing weight, but I was 7 weeks postpartum when I found the swollen node. I did not have night sweats, and I didn't have anything glaringly out of place on my CBC (complete blood count).
Urgent Care and ER doctors aren't bad. They're usually either Family Practitioners or those who specialized in Emergency Medicine. But, they're accustomed to seeing acute illnesses - sprains, food poisoning, heart attacks, Urinary Tract infections, etc.
I initially went to my OB/GYN because I was post-partum. He wasn't "concerned" but said to come back in a week. I went to my Family Practitioner the next day. His concern was because of the location of the node. He wasn't overly "concerned" either, but he ordered ultrasounds and a biopsy. The ultrasound came back "benign". It took 3 pathologists to finally confirm Nodular Sclerosing Hodgkins Lymphoma.
My message isn't to scare you. Chances are, he's dealing with a bad virus. But 2 weeks of fever is a LONG time. My best advice is to follow-up until you have a clearer picture of what may be causing it and a good plan for follow-up.
Good luck! I hope he gets to feeling better soon.