@TheBroonOneAndTheWhiteOne not sure if this is remotely helpful, but your DS's symptoms sound very much like what happened to me. For me it was Cauda Equina syndrome caused by a birth injury (although it was quite a few weeks after DD was born) but it can be caused by a lot of other things as well. When I was at hospital, they initially thought it could be transverse myelitis, which has very similar onset.
Both are serious but very treatable. I'm not going to say it was easy, but I had surgery and although the recovery took a while and I had lots of physio and other treatment (plus a second surgery more recently) I made a good recovery and today my leg strength and sensation is completely as good as it ever was. I still get a little back pain and have some residual issues due to delayed diagnosis - but it sounds like you and your son were sensible and sought help quickly. I, on the other hand, left it way too long because I was more worried about DD (who was born prematurely) than anything happening with me!
I've just been for a swim and DD is currently doing a very messy "art project" and giving foster kitty too many treats - so all is well!