Is there family/friends who can renew it in the UK for him? I believe it can be done online. Otherwise you might need to get proper advice from one of the specialist agents here.
Presumably your son is using a different passport to stay in Thailand? I don't think a UK passport can be renewed in Thailand without the expired passport. Your son can renew it in the UK when he goes back there.
Currently there are no restrictions between Bangkok and Pattaya. It's a popular weekend getaway for Bangkokians, as at least restaurants are open in Pattaya.
Kostja Grusnewitsch Just make sure you keep your receipt of posting (preferably EMS). I would allow up to 8 weeks before chasing it up. They have a backlog as people are understandably not wanting to go there.
As the link says, registering a birth in Thailand is unnecessary. A translated copy of the Thai birth certificate is fine when applying for the child's UK passport.
says. Not necessary to register the birth. A translation of the Thai birth certificate will be needed eventually when you apply for the child's UK passport.
It helps if it's a UK person who countersigns, but it is not essential. The Thai doctor who dealt with the birth, or your current pediatrician, might be worth trying. Edit: it seems doctors are specifically excluded from being counter-signatories, so my suggestion probably won't work.