Whenever he writes inshaAllah in English people always comment telling him to separate the three words in shaa Allah. 

There are so many misconceptions about this. If you are writing it in the Arabic language the correct way is writing it in three separate words.  In any other language if we say “if Allah wills” it is also written in three words. 

When one makes a transliteration, which means writing the words as they should be pronounced then you can write it in any way. A transliteration is a way to help one to read something.