Well maybe I'm wrong, but I think if you look in any grammar book, it'll tell you that 'since' (temporal) should be followed by a phrase representing a definite point in the past. This can be either a date, time, or a phrase in the past simple.
I've known him since 1995.
I've known her since she visited us on my birthday.
The 'ever' is optional of course.
Here, "since 1995" answers the question "since when?". We can't use the present perfect to answer that question because that tense describes the indefinite past.
"I've known him since we have eaten breakfast together"
Well what the hey, I'm repeating myself. I said all this above so I won't continue.
What did I miss? <scratches puzzled head>