The rule here is stative vs dynamic verbs. "To know" is a stative verb (we actually can't see the action), so "I have been knowing" doesn't make much sense. You can search for "stative verbs English" and you'll find a useful list. These don't normally change into continuous form.

The best was to say this is, "I have known him for a few years."

Some perception, state, and cognitive (brain activity) verbs don't like -ing. The only occasions they actually stand it is when their meaning change: - I'm having breakfast -> I'm eating. - Stop being stupid -> Stop acting stupid. You can't be knowing. Either you know or you don't.

The verb following "I have been" must be a sustainable one. "know" is instantaneous verb, which means you can not KNOW something for a duration of time