It sounds like your younger kitty needs another youngster to play with so he will leave your senior kitty alone.

I have the same problem with my Frankie. When I first got him and he started being rough with my 2 senior kitties, I used a squirt bottle to squirt a little water on him when he got rough with any of them.
Now all I have to do is grab the squirt bottle and he knows to stop what he is doing.