Julie Welcome to the forum.Please try not to give up on Webster,he has already had a very traumatic life form what you say.you have said it yourself he will be shattered to loose you.Does he only do this at meal times ? how does he act towards your nephew at other times ? Does he only accept females ? A little more info would help
As for the piercing whistle, is it only when your out of the room ? Do you react to him when he does it ? Ignore the whistle,dont react in any way at all,dont give him any attention until it stops,when he has stopped then praise him,tell him what a good boy he is etc..He has to learn that making this whistle will not get him your attention,it will take time & patience but please try to hang in there