so the sentence "he is always a lot" means "he is not here quite often".It is right?

"a lot" is a synonym for "often" or "frequently", although "often" and "frequently" only pertain to events happening in time whereas "a lot" could also refer to quantity of things (e.g. "A lot of food"). However, in this context "I am on frequently", "I am on often" and "I am on a lot" are all corrrect. Also the negative of "always" is "never". If you said "not always" or "always not" a native English speaker would know what you meant but it sounds awkward.

It means that "I am on-line frequently" The phrase "a lot" is similar to always When a person says "Iam on a lot" that means that it not be hard to find them. Hopefully that helps