I think if he had downs you'd have seen in the scan by now via the measurements - I don't think that will be it.
Reading about Polyhydramnios online is so tempting but the worst thing you can do, I did the same and terrified myself! It makes it seem a bit bleak but after going on forums etc I've read a lot about others experiences and the worst scenario (out of hundreds) that I've read was either premature birth (of which all the babies except one lady with twins who had them at 23 weeks and sadly they both died as they were just too small, all the others were absolutely fine after they came out of SCBU), or a couple were partially deaf. If this is the worst case scenario after everything I've read I can live with that. Obviously we want our babies to be as healthy as possible, but the chances of them being absolutely fine seem pretty good to me.
You'll be told plenty more at your next scan from your consultant, but as I said, 50% of cases are for no reason at all - which if your other tests have ruled out anything obvious I think you'll probably fall in to that category too! My mum said she had the same condition with my eldest brother, and other than being born breach, he was delivered at 40+5, perfectly normal, and healthy weight (and still is today at the grand old age of 44!), but the doctors said they'd never seen so much water when it broke! So it could just be one of those things 😊 I'll keep my fingers crossed for you, please let me know how your appt goes - I've got a midwife appt on Wed for 31 weeks but no other follow up scan to check my water levels booked for after that yet, so I'll do the same and keep you updated - and hopefully we can put each other's minds at ease for the next 10 weeks! X