Do I need to separate my Bettas? How can I encourage/induce mating?

Hello! I have an 18-liter nano tank that has had a female Betta Splendens in it for two months.

Then I took her out, and my male betta moved in. Even though there were no females left in the tank, he immediately built a nest overnight.

I then added two females, but at the moment he's just chasing them around the aquarium quite aggressively. He approaches them slowly with his gill covers and fins raised, and as soon as they try to flee, he races after them until they hide.

Should I take the females out again and wait for signs from them? They don't show vertical stripes yet.