Ever since is not really used in the negative. You would be more likely to say:
"I have never seen him since."
"I haven't seen him since."
In either case, you would need a previous sentence specifying when the "since" is referring to:
"He won the lottery. I haven't seen him since."
If you don't have a sentence before, then you can place it within the same sentence.
"I haven't seen him since he won the lottery."