It seems that the male lost his confidence/territory.
If the extent of his confidence is so low that he is now to scared to flare at anything then if I were you I would try to increase the males confidence and aggression first by isolating him and not letting him view any orther bettas/ fish for a while. This is to allow him to reclaim his territory and not feel like there is a stronger betta who is constanly trying to scare him from it.
Then, after isolation, let him see a weak, small, submissive female but only for a very short time (do not put her in the tank, just next to it).
Do not give the male any oppertunity to give up and hide, if you think he is flaring nicely then take the female away and let the male "win the fight."
If however you notice him getting less aggressive, take the female away immediately. It is vital that you do not give him any opertunity to give up and lose the "fight"
Do this a few times a week but not every day.
Eventually after this "training" your male should be brave enough to defend his territory if all his "encounters" with bettas have resulted in him winning the "fight"
This is how I make my cowards braver