Introducing a young male cat into a household that already has two senior female cats

Hi everyone
I need some advice.
Almost six months ago, my husband and I adopted a male kitten/cat (12 months old). We already have two senior female cats about 14 to 16 years old). After all this time, only one of the female cats is doing well with the new male. The other female, that happens to be the smallest, is having a rough time.
The male is being very aggressive with her, he is big and strong and he senses her fear. He has made her urinate and deficate on herself, so we had to separate the two. SO, we now have the female in a room, with a screen door.
I have Feliway plug in's around the house.
The male will go to the screen door as if to hunt the small female.
I want to get these cats together without them fighting or chasing one another.
What can I do.
The male is put in the basement at night so that the female can be out in the house over night. That also seems to be an issue with the male. He now hides under our bed at night, because he knows that he will have to go in the basement to sleep.
Help!

Honestly? I don't have a clue, been dealing with that issue x 30+ yrs with cats and haven't solved it yet except to be absolutely vigilant. Young males go after older cats, partic. females, and all the yelling, separating, squirt guns, etc. etc. make no difference whatever. Five mins. later, they're back at it. All you can do is keep her (them) safe - because he will do it to the other one in time too. Crazy as it sounds, it's worse just before and during a full moon, but doesn't go away at other times. Good luck.