Hi courtnam and welcome to the forum. Sorry to hear Marley is sick. Let's see what we can do to help him.
Have you been able to monitor his poo? If he hasn't been pooing then the bloating is most likely due to constipation. However, there's a strong possibility he has a tumor, partially because of his age. If that's the case, then all we can do is make him as comfortable as possible.
For starters, let's go ahead and put him into 3 tsps of epsom salt per gallon. Epsom salt can be found in the pharmacy section or in any drugstore. Make sure it has no dyes or scents. This will help keep his system moving as well as leech out any fluids that may be building up from an internal bacterial infection.
The best treatment for his fin rot would be API Furan 2. If you are able to find Seachem Kanaplex, you can combine the two medications and this will treat both his fin rot and a possible internal bacterial infection.