Is there one of these posters in his establishment?
According to the Civil Rights Center of the Dept of Labor, your sign is only applicable to the following:
"CRC is committed to providing its customers with clear and easy-to-access information on how to comply with federal equal opportunity and nondiscrimination laws and regulations that (1) prohibit discrimination in DOL funded programs and activities, and (2) prohibit discrimination on the basis of disability by certain public entities and in DOL conducted activities."
Note it is very specific to DOL funded programs and activities as well as some disability (ADA) discrimination by public entities.
From what I can find, and unless you can provide evidence otherwise, the gun store owner is not subject to this unless he's discriminating against hiring people or otherwise discriminating against his employees.