@SurpriseSparDay I think by saying now in the books Will is assumed to be white (purely because most people in England are) you really mean that you assumed Will to be white?
The only element of his physical colouring given in the books is that he has dark eyebrows and hair, and dark eyes. So he could well be black, or biracial, or have one of a range of ethnic origins. I didn't assume him to be white and was pleasantly surprised to see a young person of colour cast in this leading role.
Regarding your white piano teacher / black boxing coach point: there have been a number of changes made to the storyline in adaptation for the screen, on reason being the need to "show" some important themes and elements in a way that doesn't involve long narrative description. It's key to Will's character that he is fierce and brave, so swapping the piano lessons / teacher for boxing lessons / teacher at this introductory point makes a lot of sense. It also allowed for a scene of conflict and physical fight with another boy to be shown at the same time we can see that he has a relationship of trust with this coach. So lots of important themes introduced very neatly in a few scenes by making that change.
As for your concern that casting the boxing coach as black suggests that the only person Will can trust is another black person - I honestly think that makes no sense. Black men in this country suffer an appalling amount of stereotyping from some quarters. Is that perhaps where you are coming from in your worry about a black male actor being cast as a person of trust?