Looks like Bronzecat (not surprisingly) has it covered above: antibiotics, Epsom Salt bath, ensure the temp is at 80 degrees.
Might be worth a try - he might also have worms, which could produce a greatly swollen belly and damage resulting in a simlar condition.
The article given below has meds suggested which are available in the UK.
If he's still active and eating, and doesn't appear to be suffering, treatment might be worth a try, even if the chances are typically low.
http
/www.thetropicaltank.co.uk/hddropsy.htm
Dropsy
Symptoms
This disease is characterised by a swollen or hollow abdomen. Swollen areas may exhibit a 'pine-cone' appearance caused by the fishes scales sticking out. Fish may also appear off-colour and listless, and may stop feeding. The swelling caused by this disease may often be mistaken for a pregnant or egg-carrying fish. Gouramies and Cyprinids (barbs, danios, etc) are prone to this disease.
Causes
Dropsy is a condition with several possible causes. It may frequently be caused by internal infections by a number of different bacterial species. Viruses have also been associated with the disease. Poor water quality and/or diet may trigger the disease. If the problem has been caused by permanant damage to the kidney, then treating the infection will not eradicate the symptoms.
Possible cures
This disease must be caught early to maximise the chance of saving an affected fish, and even then the disease is difficult to cure. Previously, antibiotics prescribed by a vet were the only means of treatment for those countries (including the UK) where antibiotics cannot be purchased over-the-counter. There are now commercially available remedies which may help. These include Interpet #9 Anti Internal Bacteria and Waterlife's Octozin. The addition of salt (1 tsp/gallon) may also be beneficial.
Dropsy is not usually considered to be particularly contagious, so it should not spread to other healthy fish - dead fish should be removed immediately however, to avoid cannibalism. However, bear in mind that there is more than one cause of dropsy, so in some cases the infective agent could be contagious. It is preferable to carry out treatment in a hospital tank where available.
(I'd suggest using Epsom salt - as recommended by Bronzecat who's the expert I'm not,) rather than salt per se - they have different actions and the Epsom Salt encourages different osmotic results suitable to the condition.)
Best of luck!