"Knowing" is not used in this way. See, for example this short explanation in the Cambridge Dictionary: https://dictionary.cambridge.org/grammar/british-grammar/common-verbs/know.
It is in present. The Czech "známe" is present. And the English sentence is in present perfect.
The past tense would be: "Znali jsme ho několik let" = "We knew him for several years" and it would mean, in both languages, that we no longer know him (he's dead or we're not friends anymore or something similar).