I plan to live in LOS with the wife's family when I finish work in a few years on a retirement visa. Does my son who was born in England require a visa for Thailand also?

Is your son entitled to a Thai passport?

His Mum is Thai. Is that a requirement for a Thai passport?

Yes Gary, your son will have an automatic right to a Thai passport (at least until he is 21 I think, although those better qualified than I may correct me on that point). Perhaps the easy solution is to apply for a passport at the RTE in London before you emigrate or when you visit Thailand in the meantime?

See also Registering a birth in BKK

Thanks for the advice. One less thing to worry about.