I think that it is unavoidable to stop people shortening peoples names. My cousins
Lewis and
Abbie get called
Lew and Abs by their friends. My aunt and uncle can't understand why their friends shorten their names, but they do. I get called Haz or
Haze for short which I hate but people still do it.
You could try shortening is name to Nath yourself, that way he will get used to it and other people will here your calling him that and do the same.