A 3.5 gallon aquarium is just fine for a single Betta male.
When I raised them some years back, I bred the fish in 2 gallon tanks and kept adults swimming happily for years in tanks from less than 1 gallon to as large 5 gallons. They simply do not need much space.
In the wild these fish inhabit backwater pools, river overflow areas, and shallow spots where the water quality is so poor that you wonder how they survive. They have a chamber behind their brain in the back of their head that allows them to take in gulps of air and use that air directly to supplement the oxygen they get through their gills. They do not need a great deal of oxygen in the water, so they don't need "bubblers" or ancillary oxygenation and such.
Their territories in the wild are often measured in inches. They do not need a large aquarium and, in fact, tend to do better and live longer in a more restricted environment. This is what has allowed th to become such popular and decorative pets.
By the way, your aquarium looks nice as a habitat for a Betta. What you are doing is maintains what is now known as a nano-aquarium. These have become quite popular as modern filters and lighting options have improved.
I would add a couple more sprigs of floating plants to lend a still water effect. If they are live plants, they would also increase the oxygen near the surface and remove carbon dioxide and other wastes in the water as they grew.
As others have said, adding other fish is unnecessary, but the waste produced by adding a couple white clouds would be minimal-especially if you are careful not to over feed the tank. Use floating food to feed the Betta only what he will consume in 5 minutes and feed a food that slowly sinks to the white clouds such that they also clean it all up in just a few minutes. If the glass tends to grow green algae, such that you are scraping weekly, this is good news as it tells you your water is good (healthy) with no ammonia and that harmful nitrites are being converted to safer nitrates the algae can use. If you don't want to scrape, pick up a small (2-inches or smaller) catfish from the Plecostamus group and it will keep the glass fairly clean for you. Plecostamus are armored, so your Betta will do no damage to it and they have a gentle disposition, so they will not be likely to bother the Betta as the Pleco grows a bit.