Hi, Gold-Fisher,
sorry to hear your Betta's a bit off, but you've certainly received excellent advice.
I was wondering, though, if you'd ever tried or could get hold of Epsom (Magnesium) salts, which are excellent for encouraging the absorption of water and causing Bettas to poop like the veritable wind, so to speak.
It's been a long time since I've used them, and can't remember amounts, so just did a fast search:
http
/www.fishforum.com/viewtopic.php?t=9654&start=30
Constipation
Symptoms:
Slightly large abdomen, no feces are excreted.
Causes:
Incorrect diet or overfeeding. What food are you feeding your fish? How many times do you feed your fish? How long does it take for the fish to consume all the food? It is recommended to minimize the food rations by once or twice a day for fish in their adult stage. A one day fasting per week will certainly compensate for the rest of the days of feeding them. It must be noted there are certain fish species that may eat the food but which is totally not appropriate for them.
Treatment:
Green peas are recommended as they serve as a laxative allowing the fish to excrete their wastes blocked inside their bodies. Always remove the shell and if intended to be fed on fish with small mouth, mash the peas to smaller pieces.
Another alternative is the use of half a level teaspoon magnesium sulfates (Epsom salts) per 4.5 liters. If the fish recovers, improve its diet and change feeding routine.
(End of quote - looks like a good site for generalized info.)
Speaking as one who knows nothing, if I'd been feeding a fish a lot of freeze-dried food, and had him in a small tank receiving complete water changes, I'd add the epsom salts to the tank and leave in overnight, depending on results, and observing frequently, and I'd feed ONLY several tiny bits of pea about the size of his eye twice a day, if he'd take them, for a couple a days, and feed peas weekly from there on in.
(Mine get a pea-feed day weekly, rather than a total fast.)
And I wouldn't wait either, assuming he'll eat, as they have a laxative effect, in case that spot's ich and you plan to use a regular aquarium salt treatment, which would likely worsen the constipation problem.
I'd also feed frozen (moist) bloodworms and not too many freeze-dried foods or pellets, unless soaked first, and I'd also try variety in the diet, feeding several different things on a regular basis.
If the spot on his fin is white, it's likely ich??? Does it look like that might be it?
He may be a little run-down and susceptable to such things what with being backed up and all, and will certainly need treatment for that, probably salt and increased temps, but aquarium salt tends to drive moisture out of fish and may aggravate the consitipation issue.
Both conditions are imperative, but the constipation needs to be addressed first, as he'll be accumulating and resorbing toxins from the retained waste in his body, so should be ASAP; as you know, if it's ich, the nasty little critters will exponentially explode in numbers very, very fast.
With any luck, the experts are still around to advise on this, but the epsom salt thing is the sort of emergency med hardly ever used and therefore not always there on the top of the head/tip of the tongue when the need arises.
I'm only able to say what I'd do, and I'm certainly no expert, but this is based on advice which made sense to me and has worked for me in the past.
Good luck, and I hope things are much betta soon.
happy 
