I think the first email is to make the parents aware of the issue and ask them to help you come up with a possible solution since 1) they know their kid and his personality best, 2) you are not demanding discipline, rather, you are asking for a way to come up with a solution collaboratively, and 3) this would show that you respect them enough to ask their opinion on the matter. This will make them feel special, and ideally, more open to helping you solve the issue. I hope this is helpful!