If I put a house in an irrevocable trust for nephew with his father as trustee, what should I do about the title insurance policy I have on this property?

A title insurance policy is typically effective for as long as the person who acquired the property continues to own it.  In this case you transferred title to the property to a trustee.  This may cause the policy to be no longer effective.  However, it would be worth taking the time to call the title insurance company that issued the policy to inquire as to whether the policy can be continued in effect and assigned to the trustee as title holder.  Title insurance companys do not permit this when a property is sold to a new owner.  However, in your case, the property has been transferred into a trust that you established for the benefit of your nephew.  You should ask if the company will allow the transfer of the policy under these circumstances.  If they allow it, they would probably charge a fee for the service.