Not every piece of good airmanship needs to be proscribed by a rule or regulation! How about pointing out to him that he has no way of knowing whether ATC just asked someone else to ident before he came on freq, and thereby could be causing confusion?
I'm sure ATCOs on here may have other reasons/examples, whether or not there is a 'rule' about it!
I suppose you are not operating the SSR in accordance with ATC instructions, perhaps that's the route to take?