Yes Epsom is different and you can use 1tbsp which is = 3tsp/gall :)
If you can do daily water changes or you can do 50% alternate with 100% daily with 3tsp/gall of epsom.
Keep eye on him. If you will see him pooped check make sure it not white, clear, broken up white,with some transparent parts here and there would mean he has internal parasites or internal infection. Or if his behavior change or he refusing to food then definitely treat him with medications.
Since he didn't pooped than it might be a reason, and since he was on flakes and bw before which can cause of the constipation. Not sure but i would wait on meds again without being sure what he has, especially that you already used one kind of meds. I would buy meds in case you need to treat him right away.
You know i found the other thread betta with the same problem . He looks like your betta. Its wonderful thread and a lot of advice do you want me to post it for you?