Does anyone know anything about adult cat behavior?

Abigail

HELP! My 2 y/o adult male cat has recently become very skittish and jumpy/afraid. He used to be very much an alpha male! Now, if I pet him when he’s not expecting it, he jumps so high and would probably attack me if he didn’t know it was his mom. I’m afraid my BIL is abusing him when I’m not around but I’m not sure how to prove it. I just want my baby to be back to normal.