I knew or I've known (him for years)?

I knew him for years. - This is correct but is used if the person mentioned has died.
I've known him for years. - This is more accurate. It shows that the person is still alive and you still know him until now.

However, can we use "I've been knowing him for years" (Present Perfect Continous Tense)?

Present Perfect Continuous Tense is used to show that something started in the past and has continued up till now.

Therefore, formally, it's correct to say;
- "I've been living here for years"
but not, "I've been knowing him for years."

WHY???
It's because there are some verbs that we do not usually use in progressive (continuous) forms. They are mostly verbs of sensations and mental states.
For example;
 like, love, need, prefer, know, seem, mean, own, hate, want,
wish, desire, see, hear, feel, notice, understand, remember, forget, believe, recognize, appear, possess, contain, consist, etc.

Of course you don't say, "I'm liking the food" when you can just say "I like the food" or "I'm loving/believing/knowing you" if you can just say "I love/believe/know you".

ANYWAY, AS THE VERNACULAR (SPOKEN LANGUAGE), THOSE EXPRESSIONS ARE SOMETIMES USED AND ARE ACCEPTABLE.