Oh no! That's terrible!! I'm so sorry for him
I think you should be keeping an eye on your other fish in case they'll show symptoms... it might be bacterial, so look out for any bloated fish, partially raised scales or protruding eyes!

Just so you know, Dropsy can be caused by pretty much anything from bacterial infections to parasites and water quality problems so... sometimes it's really hard to figure out a reason But yeah, like you said it can be symptoms on an infection in the kidneys too, and then you can't do anything to help because it's usually gotten so far then already

And, I've heard about some easy and humane (even if it sounds cruel) method of putting a fish down, if he's suffering much from this. You could put him in a bag or other container with water from the main system, and simply put him into the freezer. You need to leave the fish alone for a couple of hours, because if you'll run around checking on him all the time you'll just cause unnecessary stress for him. This method works really well, because the fish can be in peace and quiet, and he won't feel any kind of pain before he dies. I found it when I was panicking about my own sick Oranda :'C

I hope the rest of your fish will be fine!!

Katie,
Please try an oral antibiotic fish food first before you make the choice to put your guy down!  Sometimes it is just a baterial infection.  I've seen fish from dropsey live out a pretty normal life. Some the scales never go completely back to normal and they always have that rounder apearance but at least they can swim normally again.

If he's not eating the you can treat the water with an antibiotic such as tetracycline.
I would try treating him first.  The faster he is treated the better the outcome, rather than just to watch him suffer.
Sometimes it's better just to be doing something, then you know you did all you can. Otherwise you will always wonder "what if".
He should be removed to a hospital container with aeration.

i have been trying to think of things all day, the last thing i want to do is put him down, but as i said i came back from hoilday and he was like it, i have no way of knowing how long he was ill i just feel so bad for my little guy (i know were not suppose to have favourites but he was mine) i've already asked if i can leave early but when i told my boss what for he looked at me like i'd gone out and said no

i know but i cant lie  never been able to, im such a bad liar. rang home a few mins ago and they said he was still alive. just need to run home now. dont finish for another 2 hours

Great! Will keep my fingers crossed for you both.

Oh Katie, so sorry to read all this, I cannot give you more advice than it already was given.
I keep my fingers crossed that all will turn out ok and Bubbles will recover. 

Hope all goes to be fine. Wishing a quick recovery.

Good Luck!!