Mink, it sounds like your big guy has got a nasty internal bacterial infection. This causes the organs to malfunction, creating a backup of fluids that raises the scales and causes bloat, hence "dropsy."
If your guy is eating, then I recommend Seachem Kanaplex taken internally via food. You will need to find instructions on Seachem's website on how to make medicated food using this. Another option, since Kanaplex usually needs to be ordered online, is to feed Jungle Labs Anti-Parasite Medicated Pellets. You definitely want Anti-Parasite because the medicine, metronidazole, is highly effective against internal bacterial infections when taken internally.
Definitely do what Silverfang suggests and put your guy in 3 tsps of epsom salt. This will help to leach out the excess fluids.
I hope this helps. If you have any more questions or your guy gets worse, definitely PM me. Good luck and hang in there!