The int has an 8 requirement. Can't remember the speech needed.
If you want to beat him up without killing him, try some fist weapons which deals KO damage (Boxing Gloves, Boxing Tape). I haven't tried it myself, but makes sense.
I did the INT route and here's what happened:
Spoiler
I convinced him I was a Legion assassin sent to kill him since he was a liability for the Legion. He didn't believe me at first and pegged me for a random merc/wastelander. The NCR chick comes back in and Silus tells her that I'm a Legion assassin and warns her that she can't risk it if he gets killed by me. She says she doesn't care and walks out. At this point he starts to believe you. He reveals that Caesar himself is suffering from headaches from time to time, which makes him unable to lead (The Forecaster kid from 188 anyone?). He also says that they have an officer who radio's the Legion with NCR plans every night (Possible tied in to the Control tower radio / mole quest). You then tell him he was fooled... or kill him. I fooled him with int, didn't hurt him and didn't kill him. I got caps and NCR rep for it.