How old is he? (sorry if you've already said and I missed it).
I don't know if this is the same thing but around a year ago exactly we had something similar with dd although she wasn't 'giddy' or any of those other things you've mentioned. She just suddenly lost the use of her left leg, it was very scary, she either dragged it behind her or reverted to crawling. We were rushed into A&E too and had lots of blood tests and scans and were told it might be neurological. It turned out to be something called Perthes Syndrome which strikes children mostly between the ages of 3-5 (dd was nearly 4 at the time) and is mostly to do with the hip. It was treatable and about 6-10 months later you'd never have known it had happened.
What's happening with your DS sounds a bit different to this. I just wanted to reassure you that even when it seems very scary there are all kinds of explanations.
Keep us posted, will be thinking of you xxx