Crunching Bow

My son is learning the violin using Suzuki Method and is currently practicing Lightly Row from Violin Book 1. Sometimes when he plays, the sound is.. almost crunchy. It is sometimes hard to hear the actual tune because of this crunchiness. How can I get rid of the crunching?

Without actually hearing or seeing what is going on, I would suggest one of two possibilities: The first is that there could be too much rosin on the bow, making the bow “bite” too much into the string when your son plays. Another possibility is that your son may be “pressing” too much with the bow when he plays. When you hear the “crunchy” sound you mention, you might ask your son to use his bow more “softly”, and perhaps even use a little more bow as well. But again, without hearing or seeing what is going on, these are only my best guesses.
Best of luck!