it doesn't look too severe just yet - dropsy in itself isn't an illness as such - more a side effect of something else. Is he eating? In my experience it's normally caused by something internally - whether they have worms or parasites it's normally something in their guts - at least it always has been for me.
The general consensus is there's no coming back from dropsy and euthanasia is the kindest option but that has never stopped me trying to cure it. There are meds out there to treat it but on a bank holiday weekend getting hold of them may be difficult. You could try feeding some peas to see if it's just constipated. If you have some (or can get some) epsom salt you could try a bath - by that I mean removing the fish from the tank and bathing it seperately before returning it - I use about half t-spoon epsom salt to half a litre of water and allow it to swim in the water for about 5 minutes before returning it to the tank and throwing the salt water away.
At this stage it's worth a try. If it doesn't work then euthanasia might be kinder
Good luck