Does my son need to enter Thailand on his Thai passport or is his British passport sufficient?

My son was born in Thailand and his his mother is a Thai citizen so obviously he’s (half) Thai.

When he was 7 he came with me to the UK (I’m a British citizen) and we have both lived together in the UK for the past 7 years

Now he’s 14 he is returning to his mother in Thailand to finish his studies at an international school.

His Thai passport has expiring a few years ago

He has years left on his British passport.

My situation and question is this

If he returns to Thailand on his British passport, he will get a visa exemption at immigration.

However, he has a Thai ID card and in my opinion he doesn’t need a visa once the exemption expires because he’s a Thai national and once in the country that’s it.

But his mother thinks he has to come in on a Thai passport otherwise he will end up needing a visa.

Am I correct?

Or is his mother correct?

Thanks 🙏