Of course you can talk in English to any Thai person.
The earlier you start the boy will just naturally pick up what English you know.
Just talking to him everyday for a hour or two build a relationship by just hello, goodbye, please, thankyou, if by about 3 or 4 years of age make it fun to write and speak ABC etc and just build on more words.
If he wants to learn he will if he doesn't he won't.