[X] Plan Throw the Book at the Wannabe God
-[X] Explain to him that the Imperium has laws governing organized worship. An unorganized cult will usually be tolerated, as long as there are no signs of worshiping the Lower Planes or the worship incorporating illegal acts, but if he wants to name priests, have temples or similar things, he will have to submit his teachings for review to the Imperial authorities, i.e. you.
-[X] Also explain to him that Yss disdains 'False Gods', what the snakes criteria are for that label, and that it would probably best for him to speak with Yss directly to avoid problems. You don't think there will be any, but you would prefer to have this hashed out before he settles down, in case that there is friction.
--[X] Make sure he does not take this as a threat, but do make it clear that Yss is a longstanding ally of yours and that he should not expect any backup from you if there is conflict between the two. If Yss has a problem with him, it would be better for him to leave.