afcjan, I haven't actually been through this situation although my daughter does live abroad with her family and we do see them fairly often.
I can well understand that you will miss your son but, as parents, we know the day will come when our children want to move away, and there is nowhere further than NZ. However, nowadays you can have constant contact by 'phone, by internet and even see one another by webcam. My OH and I have been to NZ and it really is a beautiful place and, if I were fit, I'd go there again in a heartbeat.
Try to think of the positive things about your son's move because, deep down, you know it's what he wants to do with his life.
I hope you don't think I'm being flippant as I certainly don't mean to be as I totally understand your anguish.