He himself say he put the sign up in anger to start off with. Decisions made in anger are often poor decisions.
Of course its discriminatory, and he knows it, despite all the feeble rationalisations.
A sign saying 3 or more school kids is not discriminatory, since it makes no judgement as to gender, culture or colour. Nor is it actually banning entry to the store - just restricting numbers.
In the same fashion the shopowner putting up a sign a few weeks ago saying no more than 2 MPs at a time is probably ok.