^^ Disregard everything in this post... it is poor advice.OK. I notice you are changing the water every 3-4 days. That seems a little too often to me. It could be you are over-stressing the betta. My experience is the less stress I create, the better the betta usually does in his environment. They really like calm and peaceful surroundings.
I was going to suggest a 100% water change with filtered water that is properly conditioned (ie. all chlorine removed, etc.). Filtered because a lot of nitrites and nitrates can be in city mains water and its possible you are having ammonia spikes. It couldn't hurt to do a 100% change with filtered water and then let it sit for 10 days, very carefully removing an amount of water every day and replacing again with filtered/conditioned water in a way as to not create stress (ie. dribble it in very very very slowly). Filtered means something like a Brita, not bottled water.
It could be your betta is a bit high-strung and needs more time to recover from stress. Don't under-estimate the effect it can have on a betta.
- ken
I'm really a bit shocked that you are still having problems. You have done all the right things and are a very attentive owner.
He certainly does not look healthy to me... As Colin said though, there is certainly a chance that you got a betta with some major problems that are proving to be nearly impossible to cure. The only thing I can really suggest is keeping his water clean (as you are doing) and give him a while off of any meds. He could be overly stressed from all of the treatment, and his immune system may not have the ability to fight off disease anymore. As you know, he probably has developed some sort of tolerance to the meds you have already used. Over-treatment can be just as detrimental as poor water conditions when a betta is ill and has not responded to meds.
If it were my betta, I would keep up with water changes and make sure he has some hiding places. No meds for the time being. Giving him a healthy environment is really the best thing you can do at this point.
His condition (which does not appear dire btw) is certainly not due to lack of competence and care. Keep up the good work and best of luck
edit: btw, i appreciate the detailed info and you ruling out a lot of factors before posting. the detailed pictures are extremely helpful. it's unfortunate that there is really no definite "solution" (as far as im concerned) to the symptoms. i fully understand being attached to a betta and wanting to cure them at all costs (been there). Hope he pulls through.