'ow' spelling dilemma

Joe asked me this last week after his spelling test. He knew immediately that he had made a mistake even before teacher returned his book (today).

He said....if c-o-w is cow and n-o-w is now, why isn't m-o-w mow???

I guess English words are more difficult and complicated than the Malay language which is mainly writing exactly as you hear it.

Nevertheless, I encouraged my boy and told him he made good use of his ears and common sense in trying to figure out the words.