Here is a good tip for your betta:
1. If you can, buy him a larger tank. It will help make the ammonia less concentrated and mean less water changes for you.
2. Even if you cannot, do a 100% water change for your fish. Make sure you scrub everything thoroughly from the tank walls to the gravel and decor. Do NOT use soap and make sure to slowly acclimate him to the new tank temperature.
3. Monitor your fish for a few days. He may be suffering from poor water conditions or some other problem related to the tank set up. Make sure he only has silk plants, a heater, and safe hiding caves that have openings larger than a quarter or else he may get stuck and hurt himself.
4. Keep regular changes each day. After a while he should perk up.
As for keeping him healthy, I recommend buying a water test kit so you can make sure his ammonia, nitrite, and nitrate levels are not too high. Also, get a good conditioner like stress coat from API or Prime from Seachem. Give him a regular schedule and nutritious food that has at least 38% crude protein. I recommend pellet form so that the food does not pollute the water as much and because freeze dried food can cause constipation and bloat.