I don't know why the Luby's would have had such a sign, at that time Texas did not have concealed carry.
Dr. Suzanna Hupp fought legislation to impose more strict firearm laws in Texas, She also fought to have the Texas law repealed which kept her from legally bringing her fire arm with her at that time in October 1991.
At the time, people were not permitted to carry a weapon into public places.
"Hupp said she realized she'd obeyed Texas law and left her gun in her car. (this was her response why she left her firearm in her vehicle) At the time, people were not permitted to carry a weapon into public places."
I remember this issue weather she did it on her own I don't know but I feel she was instrumental in getting the law repealed. I'm sure if I wrong I will be corrected but this is what I have read about the issue. There is a good youtube video featuring her and her speech to the legislators.
As for patronizing an establishment with no gun signs you can't change their minds like stated, you either ignore them or best to take your business else ware and let them know why you did.
Maybe a idea would be for several groups of people to make large party reservations at one of these establishments then when you get there go in and cancel and tell them why after pretending to see the sign. Do this for severial times a night and maybe they would take the hint as you have tied their tables up. Large parties they reserve the tables up to 1/2 hour before you get there so they can make sure they have them. Course I don't know if where you live if they would try to charge you something.
As for your son ask him if being LE that every establishment he has responded to a armed robbery, if he has, they didn't have a no gun sign and has never responded to a robbery at a establishment that they did have one. Or if he has ever heard of a shooting in a gun free zone, kinda like a drug free zone where no one ever carries drugs.
Like stated you will never change the mind of an anti until something happens to them and most of the time not even then as they like to blame a inanimate object, rather then put the blame where it belongs. I have proof about a inanimate object, I have a .45acp that has set in a drawer for over 40 years loaded and has never hurt anyone.