Ever since is not really used in the negative. You would be more likely to say: "I have never seen him since." or "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."

They do have the same technical meaning, although "ever since" implies a long time ago and perhaps that you might not expect to ever see him again

you are right, thanks ;)

"Do these sentences have the same meaning?" <--- this is the structure you need for questions involving actions. Do - (subject) - (verb) ...?