We divide speech acquisition into 2 large areas.
1)Receptive speech and 2)Expressive speech. Your grandson seems to be OK at receptive speech. Why, I'm not sure, but as long as you are talking and he evidences understanding, do not worry. Also, if he is also hearing Tagalong(?), it is well-known that kids who are learning more than 1 language at a time have delayed expressive language.