Male cat attacking female cat

Question
I have four feral cats.  They have been great and turned out to be nice pets.  Three of the cats are mom and her two babies. The fourth cat was just a stray.  From the beginning the fourth cat, Oreo, started on my mom cat, Mama.  He had not been fixed yet and Mama had been, so I figured he was trying to mate with her. He actually ran her off and we lost her for a few months.  She was in the neighborhood, we had seen her often.  We finally got her back and everything was good until Oreo found her.  He had since been fixed, but I think he still feels like he's not.  He will not leave Mama alone.  Recently he took a big hunk of hair from the back of her neck and on her face under her left eye.  I think these are the the most serious injuries so far.  Mama defends herself, but most of the time Oreo blind sides her and jumps on her.  Other times he just wanders around where she is.  Why is he doing this?  I know male cats still think they can mate after they have been fixed, but his seems a little extreme.  We've been using a spray bottle if her gets near her.  But we're not around all the time to make this effective.  These are adult cats.  Oreo is 2-3 years old and Mama is 3-4 years old.  Never have problems with the other two boys.  Is this because they are her babies?  The three boys get along fine.  I'm really worried about Mama and that she may get seriously hurt.  How do I cure this behavior?  Is there a cure?  Why won't he leave her alone?
Thanks for your help.

Answer
HI,
While my expertise is in cat grooming, I can offer my opinion as to your current situation.  The younger ones get along, as you have noted, because they all grew up together with their mom.  If this were my situation, I would be looking for a home for this aggressive male.  There is a chance that he may someday hurt her beyond what is medically fixable.  Sometimes aggressive behaviour is not completely stopped after neutering.  You may want to check and see if there is an expert on this site that deals with cat behaviours, to get better insite to what is going on with this situation.
Thank you for your interest and best of luck with your little feline family.