Translation:He is not my friend, he is my acquaintance.
since the Polish uses "moim" in the second part, I would say my acquaintance and not "an" acquaintance
Judging by the "znajo" in "znajomym" I'm assuming that it just refers to someone you know, or have met. Is this correct? Does "znajomym" exclude friends?
"znajomy" is "someone I know". Friends are included, but... they are just closer, right? They are more than "znajomi".
"znajomi" is the word for "friends" on Facebook, which itself tells a lot. Usually the list of people I know, but not necessarily know well.