He's an intact Male.
Just because she is spayed doesn't mean he isn't interested. (Think about women who have hysterectomies ~ they can't have babies but they are still desireable to men.)
He has probably "taken a liking to her recently" because he is starting to feel his sexual oats.
I have no idea how people who keep their males intact discourage that behavior (I spay / neuter) but I am sure others who do not neuter will offer some advice.
