I do have an idea, but it doesn't really involve an offical initiation ritual.
This assumes he hasn't made his required asassin-joining-kill yet:
To join the asassins, he has to kill a specific individual, pointed out to him by a member of the asassins.
Play out the asassination as an adventure, and have the (very high level) asassin follow him. (make spot checks for the PC and hide checks for the NPC if necessary, but using an NPC that is high enough not to even have to make checks against his spot would be ideal)
Right after he has successfully killed the target (assuming it all goes according to plan and there is no one alerted) the NPC 'steps out of the shadows' and simply welcomes him to the asassins, paying him the fee on the victim's head.
This is an initiation fully within the 'flavor' I think would fit an asassins guild and its 'rituals'. It shows how the asassins work (you kill because you get payed) and that there's always someone better than you....