Why does our cat bite my boyfriend but not me?

My BF has a beautiful 6-year-old (neutered) Maine Coon mix that he rescued as a starving stray kitten when he was just 5-weeks-old. He LOVES his kitty, and they are very close. I came on the scene about three years ago, and now our dear orange boy has become “my” kitty as well. (He actually own us though.)

The cat is affectionate to us both, but he is definitely an alpha male and is quick to use his teeth in any situation where he’s not 100% happy. He’s healthy, well-cared for, and soaks up plenty of attention and play throughout the day as I work from home. We have a quiet, child-free apartment with an outdoor terrace for him to roam and pigeon-watch/hunt for bugs.

However, he often exhibits unprovoked aggression toward my significant other. He is not afraid of him in the least, and I would dare say he loves his “daddy” so this behavior strikes me as very strange.

For example, today, my boyfriend was walking out the door for work, and the cat suddenly ran after him and sunk his feet in his calf and bit and scratched him out of the blue. My BF was not disturbing the cat, and it was completely unprovoked. This happens about once a day, and he will not allow me to use a spray bottle to correct the behavior like I used to do with my cats (who, incidentally were quite well-behaved.)

The cat almost never does this to me but on the rare occasion he does, I’ll dip my fingers in water and flick at him but will also do the same if I see him do it to my BF. Perhaps he’s been trained to do it to me but not to him since he knows he can get away with it? Or does he see my BF as a rival male in his territory? He gets especially angry when my BF does pushups or situps on the floor but never disturbs me when I do yoga except to ask for pets or show me his belly.