Has his doctor said what he or she thinks it might be?
An infection can sometimes stimulate the immune system and cause some joint swelling and pain in both children and adults.
It's good that your child's doctor is on top of this by doing x-rays and blood tests; I was diagnosed with rheumatoid arthritis at age 5 after a common childhood illness triggered my immune system (RA runs in my family). If (big if, here!) this is indeed what's happening with your son, it will be caught early and the quicker it is treated, the less chance of problems down the road.
I was dx'd in 1965 and back then there wasn't much they could do for me except give me massive doses of aspirin - which is exactly what they tell you not to give kids now! :) It's different these days, there are all kinds of medications available, and research is progressing all the time.
Classic signs of juvenile RA onset include red, swollen, painful joints (often multiple joints); low-grade fever; fatigue. Elevated CRP doesn't necessarily mean it's RA, however it isn't uncommon for a patient to test negative for RF (rheumatoid factor) yet still have all the symptoms. Blood tests are only a tool, not the definitive diagnosis.
Hopefully once he's over the infection, things will settle down to normal but this is definitely something to keep an eye on in the future. Right now it sounds like just one joint involved, so watch for the redness, swelling and soreness in multiple other joints like fingers, knees, wrists, elbows, etc.
Try not to worry too much! I know, it's easier said than done! Take one step at a time. :)
We're not doctors here and I was hoping that someone with experience in this area might come along. What are his doctors saying?