It could be very well that he's just suffering of old age, 3 years and 8 months is pretty old for Betta fish! They do get lethargic when they get older, much like humans. Something that you might look into are floating plants or at least one of those Betta hammocks so that he can rest at the top without effort. Right now looks like he's just having a hard time with swimming.
You said he had bloat? Do you see his poop at all and do you siphon out the water or scoop it out when you change it?
Something that would definitely help him would be to change his diet, if you can you should get either Omega One or New Life Spectrum Betta Pellets. Both are very high quality but aren't terribly expensive. The thing with foods is that you want to look at the first three ingredients, those are what the food contains most of. So you want to watch out for things that are "fillers", those are all Wheat, Corn and Soybean products. They are bad for fish in general so you want a food that doesn't have 2 or 3 fillers in the first three ingredients. Omega One is the better one as it has no fillers in their first three ingredients and NLS only has one.
So anyway, switching him to a better diet will help with his bloat and his digestive system over all. It's the difference to eating TV dinners all the time (even though you make like them) and eating a home cooked good meal with the three main components; meat, veggie and starch. So especially in older people and fish, higher quality is really essential.
EDIT: as for plants you can QT if you like but I throw mine in when I get them. I usually rinse under some tap to get out old roots or dying leaves. I undo from any cotton wrap or lead wrap it has to separate them if it's a stem plant. I give the roots a bit of a trim (leave still 1.5 inches or so if you can) and then I plop them in!
**Note** some plants are prone to melting or losing leaves when first introduced to an aquarium but don't worry they will start growing back leaves in a few weeks, just be patient. However if you see the leaf turning brown, then pluck it if you can because brown = dead.