I did have a guinea sow who lost control of her back legs, suddenly. It's a long time ago now, so long ago, I don't even remember which guinea. Babe? Emily? I think Babe, so let's call her that. My vet checked Babe's eyes, among other things, and thought it was on account of a stroke. Her (my vet) idea was to let Babe be and see if she recovered the use of her back legs. It took a number of months, in which Babe slept a lot, altho she ate too (I didn't have to do any syringe feeding) and she did eventually recover the use of her back legs and went back to 'normal'. As I say, it's a very long time ago now, if it was Babe, then 2003/2004 and vets probably do things much different now, but still maybe this can give you some hope?
It's good to hear Oscar is obviously a thriver! Good boy Oscar. Keep up the good work, you and your mummy/slave.