Aokashi I just stumbled across this thread and wow what a sad boy. I am so happy you have taken him in and giving him the best care he has probably ever had. I have had rescues just like that, my method for those really clamped fins the kind that looks all waxy and still is to do 100% water changes daily, adding aquarium salt, Stress Coat and believe it or not the secrete weapon is Betta Revive. The cheap blue stuff, 3 drops per 16oz. You may know all this already but I have found that exact mix will unclamp in 24 hours. Get Betta Revive!
I wish you all the luck with him, he will be a cutie!