1. Leave the uk and board the flight to Thailand with his British passport
2. Land in Thailand and at the Thai immigration enter Thailand with his expired Thai passport
3. If asked which do government document he traveled to Thailand with. Produce the UK passport but ask not to stamp him in with it and stamp him in on the expired Thai passport
4. All finished if immigration agree and no need to apply for a new Thai passport
Go home live like a thai citizen with his Thai ID card