His phone goes off could mean it rings or it turns off but then if the volume is going crazy the phone must still be on. Can't get my solitary brain cell around this one.
Welcome to Android Central! Does this only happen when headphones are plugged in? If you're using headphones that are designed specifically designed for iPhones, that can cause odd behavior like this, because iPhone headphone jacks are configured differently. If it happens even without headphones plugged in, then there could be debris stuck in the jack that's causing false signals -- use a can of compressed air to blow out the jack, and a toothpick to root around.