8 year old with fever + swollen knees

My eight year old son has been running a constant fever for the last 12 days.  It varies between 38C and 39.5C when not controlled with Ibruprofen,  often feeling chilled and cold.  Both his knees are slightly swollen (particularly on the inner knee) and painful.  The pain varies in intensity druing the day, ranging from a mild ache only  when he moves to very sore even when resting. There doesn't appear to be any pattern to this.  The last two days he has started to walk more on the ball of his feet, keeping the heels slightly off the ground even when he is not apparently in much pain. He is also finding stairs more difficult to negotiate.  His appetite is decreasing rapidly, having been normal in the first week of the illness.  On one day he had pain in his shoulder but this stopped and has not recurred.  For the last couple of days he has also complained about a pain on one side of his jaw which he says stops him opening his mouth too wide. He is becoming more lethargic and irritable.  I have not seen any rashes, nor has he shown any signs of coughing, sore throat, sneezing etc.  About a week before he got ill, and for some days afterward, he did have very red eyes, but they were not painful and there was no discharge.  He also had the jaw pain for a day or two at the same stage but seemed well at the time.

I have taken him to the doctor on days 3, 7 and 10 of the fever.  They ruled out any hip problems, but seem to have no idea of what could be wrong, suggesting that it is 'only a virus'.  Whatever he has does not appear to be contagious as he is one of six siblings and the others are all well.  I am increasingly worried about him as he is not a child to complain normally, and is getting visibly worse.  What could be the cause of this and what should I be doing?  I should add that he has always been very healthy.

[sweetpea03]

it could be a virus, not sure. and you can try and aid his body's natural defense system by having him take more vitamin c, plus eating healthy in general.

[samphyr]

Update 2011
It turned out to be the start of Juvenile Idiopathic Arthritis and he now has associated Uveitis. Only diagnosed at hospital.

[margypops]

I think this is a virus and I have heard of it recently, limbs aching ,a lot of lethargy, some fever , lasts about 10 days or so , if it is he will be getting over it soon. Keep him hydrated , maybe some painkillers for the aches if hes not better soon take him back again .  let us know how he is doing ..Good luck