If its any consolation, Henry, 6 months old and just neutered, is exactly the same. Before he had the chop he was awful with Tabitha who is our old lady at 16. She's a bit arthritic too, so cant fight back as well as a younger cat like Freddie who is 8. Actually, its 2 weeks now since Henry had the chop and I do think he has calmed down a little bit, plus OH shouts at him as soon as he starts on Tabbycat and whereas before he would just carry on, now he stops and walks away. So all in all, I do think your little boy will grow out of it too.
At night, I do shut Henry away from the others, or he will come upstairs and attack Tabby and chase her all over the house. He's quite happy to go in his bedroom at night though, as soon as I call him for bed, he sits and waits outside the door. AAaahh bless him!!