V.W.
Very interesting.
I'm sorry I can't really help you figure this out. But I looked up the terms and wonder now, if this is what our son had several years ago. It came out of the blue, only instead of after a head cold, he was recovering from a stomach bug (vomiting). We were out of town and not near our regular doctor. Our son was about 5 (it's been awhile so I can't remember for sure) and just started complaining that he couldn't walk. He could, carefully, just not without a lot of pain. Only his complaint was with his legs below the knees (the calf area). We thought cramps or growing pains, but it wasn't. No pain unless he tried to walk. When nothing changed, we took him to the ER and they did all sorts of blood work, but found nothing. After spending hours at the ER (about 36 hours into the whole thing) he was thirsty so I got him a powerade from the hospital machine. He drank about half and then his legs were FINE! It was bizarre. Even his bloodwork didn't show his electrolytes being off kilter. Never had another problem, but gosh did it scare us! But he literally couldn't/wouldn't walk - my husband carried him (and my husband is not a "coddler"). This maybe sheds a little light on what went on with my son (toxic synovitis?). Odd though, that the ER docs didn't mention it to us...
Maybe your son just sprained something in his foot. I've done that myself, on the outside edge. Just stepping on something unevenly could do it... rest and ibuprofin worked for me with that. Good luck. Just try to relax, mom. And listen to your son's cues. If there is something serious going on, he will let you know... and you will know too.
Hope he's better soon!