Is it permissible for parents to beat, insult, and curse their son in front of people without a reason, and is it permissible for the son to ask them about the reason for this treatment?

It is impermissible for parents to curse and revile, because it is forbidden by Islamic law, and whoever does it is a transgressor. As for striking, it is permissible only for discipline and reform. It has stages, and one should not resort to it unless other methods have failed. The striking must be non-brutal and has conditions. A child should not be struck for abandoning prayer unless he reaches ten years of age. Parents must repent from this sin, and the child must honor and obey his parents, not anger them, and supplicate for their guidance and forgiveness.