I would try 1) getting Feliway (it's a fake pheramon that usually calms
cats. Doesn't make them dopey or anything. just soothing to them) to see if
it will calm him down and 2) remove him to a timeout room *every* time he
does this. It's dominance behaviour, but he can learn. it doesn't have to be
a "bad" room or anything. Just time out by himself in the bedroom for 10 or
15 minutes. Just be consistent with it. You could get a Feliway diffuser for
that room too. Just a thought.

Hi Jo Ann,

Just wondering --  any number of things could be behind this... how long has
this been going on?  Assuming they've both been fixed and it's not
hormones...what precedes him doing this?  Is there a strange cat that visits
your house or yard that he could see? If so, he could be taking his
aggression out on her.  What I can suggest is that if he does it and she is
getting really annoyed, don't scold or punish him, just quietly separate him
for a brief "time out." Then when you bring them back together again give
them both a treat so they can both associate something good with each other
again or play with them as a reward.  I have 5 distinctly volatile kitties
in my household and am using these myself when the need arises.  Also, could
your male cat just need some rousing playtime with you alone so he's less
frisky/aggressive or whatever?  That helps mine a lot.  Hope this helps give
you some ideas to try.
Regards,
Christine and Omar, Oreo, Midnight, Robin & Tucker