Can I just ask what sort of tank you have kept him in? I mean, was it filtered and did it have a heater?
I can't believe they told you to move the fish into a small bowl. Obviously, now he'll be even more stressed than before. Oh dear!
Really, all that was needed was to vary his diet, do small regular water changes every day for a couple of weeks and, if necessary, treat with a fin rot/bacterial medication as a last resort.
If it's fin rot then it's usually brought on by a bacterial infection caused by either stress, overfeeding or bad water quality.
Yes, feeding too many bloodworms can cause constipation in bettas (oh they do love their bloodworms though!) so, it's a good idea to give them a squashed up pea at least once a week. Giving them some live or frozen brineshrimp goes down great too, usually. My betta loves live brineshrimp the best.
Lastly, if your betta is left for long periods of time on his own with nobody to amuse him (!) then he's likely to just get a bit bored and sit around more. Doesn't mean he's not well. Just needs a bit of stimulation to get those fins whirring! They are very sociable fish and thrive on your company!
At the moment I would think he is probably highly stressed at being moved from his home, getting medicated and possibly not being in a warm enough tank (well, bowl, if it doesn't have a heater).
If that was my betta I would gravel vac his tank, do another small water change, check the water stats for ammonia/nitrite and if all ok and temp at correct level, put him back in there. If his condition has really deteriorated I'd then start medicating with something that covers fin rot and other bacterial infections (in the UK we have MYXAZIN).
And be careful not to overfeed during this time.
Hope the lil' fella pulls through!
Athena