The law relating to this and to John Terry is that it's an offence to use threatening, abusive, or insulting words in the sight or hearing of someone likely to be caused harassment alarm or distress thereby. It becomes more serious as an offence if the words are also 'racially aggravated', that is displaying hostility directed at someone because of their presumed membership of some group of persons defined by race or religion.
First question therefore is whether saying 'F off' to the lad fell within the above definition, in particular 'likely to be caused'. It would be surprising if it did. Calling a boy a boy presumably isn't. So that leaves the racial element. It's highly unlikely that he would be alarmed or distressed by being callled white or that anyone hearing it would be. So does the statement as a whole become more alarming or distressing than the individual words? Possibly, but still not 'likely' to cause distress or alarm. That 'white boy', in the context, were words displaying intentional hostility towards the boy as of a 'group', a race, which is white is fairly plain, but the case fails because of the other elements which must be proved.