Because you are his parents, you are responsible for ensuring his safety at all times. This includes his physical, emotional and spiritual safety. You have alluded to the possibility that the music lyrics he is singing with his band may not be spiritually safe for him and others.
Let him know that his reputation is affected by his choice of music, and that a good reputation is invaluable. Thus, you would like to attend at least one of the band’s practices. If he struggles with your decision to attend, it may be that he is uncomfortable with you hearing the lyrics.
Share with him that as his mom, you are his biggest fan. In order for you to continue to support his participation in his band, you need to be familiar with the music he plays.