White poop can be caused by parasites, or by stress. If it's a one time thing, assume it's stress-related. If it continues, especially if he has other symptoms, let us know.
I would start by using Epsom salt at a dosage of 1 teaspoon per gallon. (I would not go higher than this amount right now.) The brand you have on hand, which contains magnesium sulfate heptahydrate is perfect. (This is the formal "chemical name" for Epsom salt.)
I would not use Aquarium salt. It is high in sodium, which can cause bloating, constipation and buoyancy issues. These are exactly the issues that he's already dealing with.
Do not give him any freeze dried foods. These absorb moisture from the system, leading to constipation.
Also, do not feed a pea. Bettas are insectivores. Giving them quality protein-based foods should be enough to get their digestive system functioning well.
Aqueon food is decent, but it contains a lot of grains/fillers. These can cause bloating, constipation and buoyancy issues in some fish. Instead, I would switch his food to a good protein based pellet, such as:
-- New Life Spectrum
-- Omega One Betta Buffet.
If this was my fish, here's what I would do:
1) Prepare a "treatment solution" by stirring 1 teaspoon Epsom salt into 1 gallon of water. Be sure the water is within +/- 2 degrees F of his current tank water. Add conditioner!
2) Remove 25% of the water from his tank. Replace it with the new "treatment solution."
3) Wait at least 15 minutes. Then, repeat Step 2. Do this two more times, until you've done a total of four 25% water changes.
This will get him to a dosage of 1 teaspoon Epsom salt, and it will remove the Aquarium salt in the process.
4) If he's in the 0.5 gal tank, then I would do a 50% water change, using the "treatment solution" every day. (Prepare more of the mixture, if you need it.)
I would fast him for 1 or 2 days. This means no food at all. Doing this will give his digestive system a chance to clear out.
Then, you have a choice:
If he's still having SBD issues, feed him small meals of frozen daphnia. It has a laxative effect on them.
If the issues seem better, feed him small meals of either New Life Spectrum or Omega One foods.
Keep him in the Epsom salt solution (1 tsp per gal) during this time.
Keep us updated.