Protection spells are useful in cases like this, but they should be used in conjunction with mundane actions to protect yourself as well.
Start by telling your parents about his threats and come clean about your relationship with them as well. That way if he does go to your parents (Which I doubt he would do.) they will already have the true story from you.
Go to the police yourself or have your parents go with you to file a complaint about his threats. Save all correspondence from him, especially correspondence that includes threats. Keep a log of every time he calls you on the phone. This evidence can be presented to the police in order to take out a restraining order/no-contact order on him. Once the order is issued continue to keep the same records. If he continues to harass you in violation of the court order they can arrest him.
Whether he still has love letters from you or not means nothing. The worst stalkers are often those who are now ex-husbands or ex-boyfriends. That you cared for him before does not stand as an excuse for threats and stalking.
Truly, if this is happening you need to take action now. Don't assume that he isn't serious. Maybe he is only doing this to manipulate you into doing what he wants. That in itself is a form of mental abuse. But sometimes (And I hate to scare you more with this.) they will go from stalking and threats to more dangerous actions. You need to take action to make sure you are safe.