Based on the instruction given to me by my CCW instructor who is a retired US Marshall and helped write SC handgun laws and advises district attorneys on how to interpret those laws.
YOU CAN LEGALLY IGNORE HIS SIGN
The only thing he can do if your handgun accidentally becomes visible and he sees it or someone sees it and complains is he can ask you to leave and can only prosecute you if you refuse to do so
Here is the SC law on the signs
SECTION 23-31-235. Sign requirements.
(A) Notwithstanding any other provision of this article, any requirement of or allowance for the posting of signs prohibiting the carrying of a concealable weapon upon any premises shall only be satisfied
by a sign expressing the prohibition in both written language interdict and universal sign language.
(B) All signs must be posted at each entrance into a building where a concealable weapon permit holder is prohibited from carrying a concealable weapon and must be:
(1) clearly visible from outside the building;
(2) eight inches wide by twelve inches tall in size
(3) contain the words "NO CONCEALABLE
WEAPONS ALLOWED" in black one-inch tall uppercase type at the bottom of the sign and centered between the lateral edges of the sign;
(4) contain a black silhouette of a handgun inside a circle seven inches in diameter with a diagonal line that runs from the lower left to the upper right at a forty-five degree angle from the horizontal
(5) a diameter of a circle; and
(6) placed not less than forty inches and not more than sixty inches from the bottom of the building's entrance door.
(C) If the premises where concealable weapons are prohibited does not have doors, then the signs contained in subsection (A) must be:
(1) thirty-six inches wide by forty-eight inches tall in size
(2) contain the words "NO CONCEALABLE
WEAPONS ALLOWED" in black three- inch tall uppercase type at the bottom of the sign and centered between the lateral edges of the sign;
(3) contain a black silhouette of a handgun inside a circle thirty-four inches in diameter with a diagonal line that is two inches wide and runs from the lower left to the upper right at a forty-five degree angle from the horizontal and must be a diameter of a circle whose circumference is two inches wide
(4) placed not less than forty inches and not more than ninety-six inches above the ground
(5) posted in sufficient quantities to be clearly visible from any point of entry onto the premises
does his sign look like this?
If not then it is not an enforceable sign.