I have an 18 yr old son and live in mrytle beach,sc. We were told by the horry county police that we cannot throw our 18 yr old out, that we must have him evicted. Is this true?

Well, if the Police won't remove him you have no choice but to evict him.

Well if you asked the police no you can't, if you had not asked them, they can't do anything after you did either.
But with that, legally you have to give him notice to move, and evict him, if he is doing something illegal or a threat you may get a order of protection and have him removed by the court.