My now ex fiance broke off our engagement about a month ago and he wants my ring back now. He proposed on Valentines Day.
As a general rule, an engagement ring is a "gift in contemplation of marriage" complete when the marriage occurs. Until then the gift is sort of "voidable." Be aware that Idaho does not have any statutory or case law specific to this question, but there is no reason to believe that Idaho courts would not hold similarly to the rule followed in nearly every other jurisdiction. Returning the ring is the honorable choice. Refusing to return it may only prove to him that you are not the person he deserves.
Best wishes for an outcome you can accept, and please remember to designate a best answer.