Hi SevyJane and welcome to the forum. I'm sorry to hear about Pink Floyd. From the picture, it does look like a possible fatty tumor. If this is indeed the case, nothing can be done except make him comfortable. Bettas with tumors can live comfortably for months.
Treatment-wise, how long did you treat him for parasites and is he still in epsom salts? If he's not still in epsoms, I'd put him back in at 1 tsp per gallon to keep fluid buildup down. It won't eliminate it completely but it will help. Also, have you noticed anything unusual about his poo? Not pooing or pooing white stringy stuff?
Don't worry about feeding flakes. The only reason pellets are recommended is because they aren't as messy but with the number of water changes you do, that's not a problem. Some fish simply won't eat one type of food. As long as the flakes are good quality, it's fine. :)