If primafix doesn't work I'd try "shotgun" approach and add erythromycin ( gram positive bacteria), & tetracycline ( gram positive & negative), and possibly triple sulfa (gram negative inhibiting) at the same time. I don't know if it's safe to do that.
But, if he's getting worse... And, all the above hasn't helped. I think I try 2 or 3 different ones combined. One gram positive killer and 1 gram negative killer. You could use or skip the inhibitor.
( I got my betta with fin rot. Battled it for about 6 weeks. I'm about to do erythromycin and tetracycline combined on my betta. Betta Revive, salt, Furan 2 didn't work on mine and I think he's getting worse)
I hope your Betta gets better!!!
(Friend of Bill W., One day at a time)