Frank and Tanja are correct - you can't say "He has already said about this." The word "say" needs a direct object. You can't just "say," and you also can't say "about" something. You have to say *something.*
For example, these sentences are okay:
"He has already said A LOT about this."
"He has already said EVERYTHING HE WANTED TO SAY about this."
"He has already said A FEW THINGS about this."
If you don't want to explain what he said, then you need to use a different verb, like "talk." In general:
We talk ABOUT something.
We say SOMETHING. (no "about")