Different people on this forum will give you different advice. What I would do is different from what has been recommended, but I feel the procedure I'm about to outline is safe, and will not harm your fish..... I would follow your instincts. If he is losing weight, while still eating, this can indicate an internal issue.
Here is what I would do, if this was my fish:
- Keep him in the QT tank so you can monitor him.
- I would treat with Epsom salt. (Not aquarium salt!) Epsom salt is safe to use. Make sure it is 100% magnesium sulfate. (No dyes, perfumes or additives.) Start with a dosage of 1 teaspoon per gallon. (I don't recall seeing the size of your QT tank, but I'll assume it's a 1 gal tank. I would treat with 1 teaspoon of Epsom salt / gal of water. So if you have a 1 gal QT tank, you need 1 teaspoon.)....
- Assuming you have a 1 gal QT tank.... Fully dissolve 1 teaspoon of Epsom salt in a small amount of conditioned water. Add it slowly, over the course of an hour or more, to the QT tank. (If you decide to use Epsom salt in the larger tank later on, it will not harm any plants.)
- When you do water changes, dissolve the correct amount of Epsom salt (1 teaspoon per gallon) into the new water.
- I would not use aquarium salt. Aquarium salt contains sodium, which can lead to fluid retention and can damage the kidneys. (Epsom salt does not contain sodium, and does not put stress on the kidneys.)
- Switch to a high quality protein-based food. The two most often recommended foods on this forum are New Life Spectrum betta pellets, and Omega One Betta Buffet pellets. The Top Fin and Aqueon foods are decent, but they contain a lot of grains. Some bettas are sensitive to grains and fillers, and do better on New Life Spectrum or Omega One.