First of all you should tell him that you are a new ruler with new objectives. Try to make a fresh start with him. It is probably not enough though. What to do?
It seems to me like you have to give something (provinces/magic item/gold/promises) to get something (peace) in this situation. Because you obviously can't threaten him as you are in a pointless war. Right? And by pointless I suppose you mean that it doesn't lead to any substitanial victories for either side. That fact however, should be used to show you have a common interest in peace.
If this isn't enough he probably feels that your nation has wronged him in greatly. He lost a lot of time and gold in fighting your nation and won nothing. The solution is of course to give him something, possible a province, if that isn't a too big price for you. Remember, you don't lose it, you invest it in peace.
Combine all this with nice manners and probably some more elements that are unique to the situation and all should hopefully solve itself.
