Suddenly aggressive male cat
Just a little background before I get to my question. I have 7 cats (2 male, 5 female), all are indoor only, and all are spayed or neutered. They range in ages from around 7 to a few months old. The newest additions are the two youngest (2 sisters) who were added to the family about 5-6 months ago. Other than the two kittens several months ago, nothing else has changed recently.
Within the past few weeks, my large male cat (17 lbs and about 5 years old) has become aggressive towards one of my smaller female cats (about 8 lbs and a year old). It's definitely not something she's provoking as she's not an aggressive cat at all. They used to be big pals too, and played together a ton. Lately though he'll put his head low, lay his ears back a bit, and basically charge at her, trying to jump on her and bite her neck. Since he's 17 lbs and she's maybe 8, it's not exactly a fair match... She's much quicker than him, but sometimes she doesn't see it coming (or she's sleeping).
Just within the past few days, my large male cat's sister (who is about 15lbs) has started to be aggressive towards the smaller female cat too. Again, they used to be big pals, so this is really strange...
I've tried using a squirt bottle to stop him from doing that, and it does stop him for a minute, until he washes off the water, but then he goes right back at it again. So I've been separating them when hes gets in that mood, since it really upsets the smaller female. She was rescued from a cat hoarders house where judging by how scared of people she was, she wasn't treated very well at all.
Besides the squirt bottle, and separating them when he's acting like that, I've also tried the "feliway" plugin thing that's supposed to have a calming affect on the cats. It hasn't done much to help so far. Separating them seems to distract the male cat long enough so he leaves her alone for a while.
Any ideas what could be causing this, and what I can do to help return things to normal? Even though I suspect he's just doing the whole dominant male thing, I'm still very worried he's going to hurt her, and it's very upsetting to her.