Get him to a fish vet or it might be too late, but 250mg of chloramphenicol per gallon of water usually cures it. ( i reccommended the vet because chloramphenicol is usualy harmfull to humans) Yes a bit of salt can help aswell because while the fish is infected its body is letting in more water than usual through its skin so the the salt can actually stop the water from going in the fish.
Heres some advice from a book:
Dropsy is a disease/symptom of internal bacterial infection in which the cavities of the body accumulate fluids until the scales tend to stick out at right angles.
There is evidence this is a virus infection but 250 mg of chloramphenicol per gallon added to the water will usually cure. When only a single fish devopes this condition, it does not normally spread.
Another book says:
Since salt water has a lower osmotic pressure than fresh, sometimes keeping a fish with dropsy in a slight salt solution will decrease the water entering the body. This will give the fishs organs time to recover without the strain of trying to expell all extra water so that it will be able to resume normal functioning when it returns to normal water.The fish should be isolated and 7.5ml (1/2 a tablespoon) os salt per 4.8 litres (8 Pints) added over a period of time. As the fish recovers, use water changes to dilute the salt solution until the water is back to normal. But if the cause is bacterial, antibiotics from a vet may help.
I hope your betta makes it