What fish can be kept in a 2.5 gallon freshwater fish tank along with a small male betta fish?

Another betta is not a good idea. They'll only try to kill each other. You can keep 5 to 6 Boraras brigittae or a group of small shrimps like CRS/Tiger/Cherry shrimps. Filteration should be optimal. You can add badam aka catappa leaves to release tannins. Will colour the water but will keep the shrimps healthy. Easy moss like java moss or christmas moss will add to the decor and house micro fauna for them to feed. All the best!

I have gotten away with keeping certain fast moving tetras with a betta. As long as they are small, fast, and dont have long flowing fins(That would attract its attention and provoke aggression). If your going to add anything tropical make sure to add a heater as well that keeps the tank at 78 degrees. I would also take a look at this article: http://levingman.com/blog/how-many-fish-can-i-have-in-my-aquarium-by-lev-ingman/

Shrimp are a good idea, as are some of the really small rasbora species, though the latter can be a little delicate, as other posters have indicated. A male betta would be fine, but just the one, and it wouldn't be a good idea to add anything else. Water quality will be the key, and that's the biggest challenge in such a small volume: consistent weekly water changes of about 20%, treating the water with conditioner to remove chlorine and other irritants, feeding wisely, and keeping a daily check on health. Good luck!

for something simple, just add some ghost shrimp. a 2.5 gallon is really too small for any tankmakes other than a couple shrimp and snails. If you had something like a 5 gallon or bigger then a pair of female bettas (no male, he'd kill them) is a really good fit or some sparkling gourami's and 5 dwarf rasbora but those are rather delicate fish. with male betta keep them on their own until you get a tank at least 10 gallons in size

None! A tank this small is barely big enough for one male betta. Ideally, one betta should have at least 3 gallons of water, 5 is even better. 2.5 will do, but you must be diligent about maintenance. You will need to do water changes at least once a week to avoid a build-up of ammonia, which is harmful and can be deadly if high enough. Also, to keep waste at a low level (since waste is what leads to ammonia), feed a good quality pellet-type made specifically for bettas. Aqueon makes one that is good and not too expensive. Some other goods one are Omega One Betta Buffet pellets and New Life Spectrum. Feed one pellet at a time, and watch to make sure it is eaten before adding another. That way there is no uneaten food rotting in the bottom of the tank. I usually feed 2-3 times daily, a few pellets each time How many pellets depends on the size. A good rule of thumb is that a betta's stomach is about the size of their eyes, which might be 2-3 pellets or 5-6, depending on the size of pellet. Avoid flakes, which are harder to keep track of and therefore tend to foul the water quickly, and don't use freeze-dried foods, even if they are labeled as betta treats. Thay tend to cause bloating as they absorb liquid in the fish's tummy. If you want to give him a bloodworm treat now and then, buy some frozen bloodworms instead. They love them! As for other fish you could put in the tank instead of a betta, it would need to be something really tiny, like Endlers. They are similar to guppies, but smaller and are very colorful and active. The males are smaller and more colorful than the females, so  you could have 3 of them.  They are livebearers, meaning that they give birth to live babies capable of swimming at birth, rather than laying eggs. If you want to have babies (fry), 1 male and 2 females might work in a tank that small, but you would have the potential for quickly becoming overcrowded.

The tank is too small even for you betta. Try to upgrade to a minimum of 10 gallons if you can. If you cant then instead of tankmates you could try adding some java moss, a simple plant to grow. It would also consume nitrates from the water and keep it clean.