Can my son travel to Thailand on a 30-day visa and extend it while waiting for his volunteering visa?

If you know which Thai embassy or consulate is processing the application (I'm assuming it's e-visa) then try finding that specific office on Facebook and sending them a message. Many of the consulates and embassies are better at replying on Facebook for some reason.

For the best odds of getting a reply, keep it to a single message that is short and precise. Include the question and any relevant information in that single message.

Something like: Hello, I would like to check the status of a non-O volunteer visa application number xxxxx submitted on 15 November 2023 for Firstname Lastname with passport number. His flight is 31 December so we are hoping the visa will be issued by this date.

Thank you

If his nationality is eligible for visa-exempt, yes, he can enter Thailand on visa-exempt for 30 days. And he can apply for 30 days extension on it. BUT he cannot do volunteer work legally when he is on his visa-exempt entry and touristm extension.

When his visa comes after he enters Thailand, he needs to leave Thailand (by air or land), enter Thailand again using his Non-O visa based on volunteering. Then he can start volunteer work permit procedure and volunteering.