I don't have the first hand experience you are hoping for, but after your posts I can see you need help and nobody has managed to see your posts. I hope the know how I have is enough until someone else more qualified can chime in.
Sounds very much like a swim bladder issue.
Every site I can find on the condition all agree that constipation and overfeeding are the most common cause in bettas and suggest fasting him for a few days (some say two days, others three)
Some suggest feeding a pea after the fasting but people on this site often speak against them so I would appreciate someone with more knowledge cutting in at this point.
Another cause is listed as a bacterial infection that is inflating his swim bladder. Appropriate medication could be applied but it is up to you if you prefer to treat for that as well right away or give the fast time to work.
On top of that his bowl changes in such a tiny space need to be far more often. Swim bladders are sensitive to infection and even if he is constipated he is still secreting ammonia through his gills that cant be allowed to build up. Partial water changes every other day and one full change a week.
I wish I could give more coherant advice but I really don't like to speak up unless nobody else does. With luck might pop in just to correct me :)
 Been doing some research on your behalf and an alternative laxative to a pea (and much safer) are frozen daphnia (water fleas). If you have a large pet wholesaler nearby or something you might like to ask? Apparently they are a treat food to bettas as well the same as bloodworms so he gets to be pampered as he recovers.