I understand how scary it it. Our son had a similar episode when he was about 5 yrs old. And it happened while we were away from home. We too, took him to the ER after 2 days, deciding that he wasn't just sore or stiff or having growing pains, etc. Our son was not in pain EXCEPT when he tried to stand. Otherwise he was fine.
ER said pretty much the same thing yours has, except they never even gave us the "transient synovitis" or "toxic synovitis" suggestion. They had no clue what it was. Did lots of blood work, etc. No ideas. But, while we were there, they did give him some Advil, and we gave him a Powerade, as by that point some of his electrolytes etc were a tad off, but not enough to "cause" anything (he'd had a cold just prior and was drinking only water and not eating normally). At some point during the waiting in the treatment room (3 hours or so?) my son tried to stand up and was just fine. No more problems. It was all very strange. This was 10 years ago.
I did pull up transient synovitis on wikipedia, just to see (again) what it says about it. And it does say that it can last for 7-10 days. It is also described as a diagnosis of exclusion, meaning that they can't diagnose it until they have excluded every other possible diagnosis. There is no "test" for it.
I would suggest, laymen here, not a doctor, that it is in fact what your son has. Have they or you given your son any medication for the inflammation? I would be sure to give him something for that specifically--ask what to give him specifically for inflammation. Maybe it's advil for children. Maybe there is something else they may recommend for your son, and hopefully, he will recover before you go forward with the MRI and it will be unnecessary. That is a lot of trauma for a young child. And it is very scary as the parent.
We were fortunate, in that our son recovered while in the ER, so we never were faced with having to do more and more and more invasive testing. Just the blood draws (which were like torture for our son as it was).
Please let us know in another day or so how he is doing.
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001976/
ETA:
According to this: http://emedicine.medscape.com/article/1007186-followup#a2646 patients taking anti-inflammatories (ibuprofen, or naproxen) basically cut the duration of the symptoms in half, 2 days vs. 4.5 days.
ETA again: Forgot to ask--Did your son have a cold recently? There are only 2 things that "typically" are reported before transient synovitis. Some sort of trauma to the hip and having just had a cold/virus. Just a common respiratory infection or ear infection...nothing special. My son had a cold just before he was affected by this. Has your son had a cold? Even a mild one? Or an ear infection?
