Where are you now? He'll need a Thai passport to enter Thailand (if you're currently in the UK) but if you're in Thailand, he'll need a UK visa to enter the U.K. if he only has a Thai passport. I'd get both passports, wherever you are.
Once the baby is registered you will still need to apply for a passport which requires all the same documents. There is no requirement to register births overseas; getting a UK passport establishes citizenship.
A simple explanation is that a "visa" is used to enter a country, that's all. Once in a country you get stamped in until a specified date, which can usually be extended. The visa is long gone.
I did because my son will most likely be living in Thailand and going to school here for a long time. I didn't want his passport full of overstay stamps and it was easy enough to just include him as my dependent
not in Bangkok. I submitted my T28 about 6 months after I actually moved (although I put a more recent date on the form), sent it off with my regular 90 day report and got both receipts back without a problem.