"Does anybody know him?"
Translation:Хто-небудь його знає?
In general хтось is more like "somebody" and хто-небудь is more like "anybody".
So, "Хтось його знає?" sounds like "Does somebody [here] know him?". While "Хто-небудь його знає?" sounds like "Does anyone know him?". Which can mean really the same as the first sentence. Or can have a wider meaning ("Does anyone at all know him") or a different feeling (e.g. more annoyed).