If he's never left Thailand, does he have a passport? If not, he can't have a visa. Tod Daniels will correct me if I'm wrong, but once he needs to leave the country, you take his Thai birth certificate with his passport to the airport and he will get an exit stamp and a note in his passport that he was born here. When/If he comes back he will need a visa. How long a non-Thai person, born in Thailand, can remain here without ever leaving, I have no idea.
Why will your passport renewal take longer than normal? renewing in Bangkok can usually be done in less than 3 weeks, sometimes it's as quick as 10 days. You keep your old passport while they're processing the new one so you can still get Extensions in country, although you're not supposed to be able to travel overseas on it.