Sorry no one responded to your thread earlier. For some reasons, it had escaped my radar
I like SBD topics (well, I don't like SBD on any fish, but it is a topic dear to my heart) as one of my boys has SBD but it appears to be a genetic defect for him.
Ok. Let's see.....
For a 1.5 gal tank, ideally the water should be changed every 2 days at 100% each, can stretch longer if you have live plants to use up some of the ammonia. But generally super clean water means very happy fish (as long as the fish is not easily stressed by water change)
From what you said, you have him for 2 months, but the SBD symptoms came on 2 weeks ago, so I will rule out the genetic theory for now.
You were right, ES would help with SBD, but one bath won't quite correct the problem so it could continue to persist. So let's put him back into ES tx.
To make things easier, pre mix 1 tsp of ES with a gal of conditioned water which you pour into his tank and keep it at 1 gallon for now.
Do 100% water change everyday with the ES tx for the next 10-14 days. Give us an update in 5 days to see if there are other signs.
Do you have any frozen daphnia or brine shrimp? That would help to make sure the intestines are not blocked or occluding the swim bladder in some way.
You want to try to correct this fast because if the SBD is not treated, it can become chronic and the fish won't be able to swim normal again.
In rare cases, it can be caused by bacteria, so you will need to observe to see if there are other changes in the meantime.
HTH and hope your betta get well soon.