Hi Just saw this thread and thought I would let you know what happened to ds1 when he was approx 3.5. He suddenly lost the use of his legs couldn't even get off the sofa for a few days and before that he fell a lot and got weaker, he was hardly eating at the time either though so was definitely ill with something too.
We spent the day at A&E being seen by various doctors and having tests. They couldn't be sure what it was but thought it was neuro problem - think a part at the back of the brain. They said the condition can often occur after chickenpox but ds had had chickenpox a year earlier. The good news for us was he regained the use of his legs after about a week but they told me it could take months.
I didn't find out exactly what had happened to him because I refused further blood tests when he was well. They took blood at the time and it was so traumatic for him I couldn't put him through it again. One thing they said was this condition sometimes follows a virus. I remember he had had the antibiotic ethromycn not long before it happened and I personnaly felt that it was not a good antibiotic for him and had noted in his red book I didn't want him to ever have this antib again.
Not sure if any of this helps you or even if your son has the same thing but I do know how you feel it was so scary and I just couldn't believe how quickly it happened to him. Good luck and I really hope your son recovers as quickly as mine did if he does have the same thing.