I might try WunderKeys with him. The material will be easier for his age, the pictures are fun and will imply some techniques that you can model instead of trying to explain, and children at that age are not expected to read well anyway. I'm assuming if they are living here they will begin to pick up some English through preschool classes or daycare.
Thinking of specific lessons in WunderKeys with "polite" sounds or "rude" sounds it would be very easy to play something nice with a smile and hit the keys a few times with a frown to get your point across.
Using google translate to just translate a few keys words might be helpful and writing them in on his books. It sounds like mom and dad likely speak English as well or at least the sister, and they could take a few moments to go over his lessons with him.