How to correct my advisor's English?
Some non-native speakers of English are eager to improve their English and grateful for corrections. Others have, unfortunately, a very different attitude.  Ahmad correctly suggests caution if your adviser is easily offended, but it may be difficult to determine whether he's easily offended. So I'd suggest beginning with just one of your adviser's errors, perhaps a spelling error where it's easy to check in a dictionary that it really is an error.  See how he reacts to that before correcting more errors.