some things are best left alone.
if you asked 2x and got the same answer then leave it. going out on a weird tree limb here, maybe it's a bully from childhood, or an abuser....maybe someone his family liked but he didn't, a weird neighbor, serial killer, just pain in the ***...what ever.

every family knows someone they would rather not...or has it as a member. that one person. your boyfriend may call this person "friend" now because that is the safest, easiest way for him to describe them to you. what ever the relationship is or was it's a painful one.

truth is he gave you the answer you asked for. not every question get YOU the answer YOU want. leave it. what ever it is it's a deep secret and he feels you are better off not knowing it.

let it go.