Sometimes dropsy-like conditions (which are really symptoms of bacterial infections within the fish's system and the swelling has to do with the body being unable to release fluids) come about because of water which isn't quite as good as it could be. It may also be that your male is just getting old for a guppy. Let's assume that for now.

However I sometimes get these guilty feelings if something is going on in my tanks. And usually, I realize, the guilt is deserved and I'd better back off the computer (where I'm urging others to change more water) and then go and change more water. ;)

(Lately there have been very few casualties, the pollution has taken it out on me as spectacular algae blooms and for some annoying reason in some cases, spectacular Cyanobacteria blooms. Nature will find some way to clean up whatever mess there is. I should be grateful for the unsightly growing "stuff" and not the loss of fish. Maybe that is a perk (mercy?) in having fairly "established" aquaria.)

Ask yourself how long you have had that fish. Was it purchased at a store and if so, how big was he then? If he grew up from a fry, roughly how long ago was he born? And what temperature is your aquarium? Life expectancies can vary quite a bit, just by temperature.

Elderly fish, just like elderly people, don't cope as well with stress and may have less effective immune systems. Just as really old people are at great risk from a strain of flu that would just make you or me miserable, a concentration of bacteria (in the water) which wouldn't bother a young guppy, may be hard on an older one.

Do a partial water change (with treated water of the same temperature, seasoned if possible) anyway. That may be more of a benefit to his tank mates than him, but it shouldn't hurt and might help.

You do want to keep his mates from catching what he has. Once something like "dropsy" takes off, it is hard to effectively "cure" it. Often aquarists, who do not wish to euthanize the dropsy afflicted fish, simply isolate it and try to give it their best TLC.) The progressive partial water changes will hopefully keep waste products down and limit bacterial populations. And because ammonia kills fish so fast, if we hit the tank with an antibiotic, we destroy the beneficial bacteria (which are processing fish wastes as a part of the nitrogen cycle) along with some of the "bad guys" and we have trouble because then the whole aquarium can become a deadly pool of ammonia. (Hence the idea of a quarantine or hospital tank on the side and rigorous replacement of that water with water from a regular tank.)

http://www.guppylog.com/story/2005/4/20/19226/2648
includes a post of mine on life expectancies and temperatures. It might be of interest to you. Please note that the numbers and research are 100% Stan Shubel's and not mine. :)