I know that he currently is keeping it in Caesar tent, but unfortunetly my reputation among Legion is very bad hence all legionnaire attack me if I come in tent. What could I do? Putting on Legion's wear didn't help, I am still recognized.
Haven't you got the Mark of Caesar from Vulpes after confronting Benny at The Tops? Normally, your reputation with the Legion is made anew at this point. If you have managed to make your reputation lower after that (the quest Beware the Wrath of Caesar! appears as failed), you are out of luck - I suppose you could attack the NCR in a few places in order to gain good rep with the Legion, but I'm not sure if they'll let you enter The Fort if your reputation is mixed.
You might sneak to Cottonwood Cove, kill Luculus, take the raft, sneak at The Fort (you will have to run away from the guards at the door, then start sneaking), go to the Caesar's tent, kill Caesar and take the Chip from his corpse. This will be very difficult. I don't think it's possible to sneak through the Fort without a lot of Stealth Boys, and Caesar is in a room with many high-leveled enemies. Don't take Boone to Cottonwood Cove.
It's easier if you restore a save from after getting the Mark of Caesar and before failing Beware the Wrath of Caesar!.