I expect there are many reasons for the under achievement of white boys - although I think this under achievement relates largely to white boys who live in deprived areas.
There are large areas of the country that are very deprived - with poor housing and transport, and poor employment opportunities. Schools in these areas have difficulty in attracting experienced and skilled teachers - they are more likely to want to live and work in more leafy, attractive areas where there are less social problems. These areas tend to have been ignored, whereas more investment should be directed towards them.
People from ethnic backgrounds are more likely to live in cities or large towns where, even when there is deprivation on an individual level, there is better access to transport and public amenities - libraries (with access to computers), museums, parks, youth clubs. There are always pockets of deprivation in large towns and cities, particularly with regard to housing, but, generally, there are compensating factors.
White disadvantage tends to be generational and leads to cycles of deprivation and a feeling passed down that education is a waste of time. This is particularly so when there are few, mostly low paid, local employment opportunities. In such circumstances, it is difficult to create an environment where young people feel motivated to learn.
Young people whose parents were born elsewhere, where things like education (which are taken for granted by the resident UK population), have to be paid for, are often brought up to take full advantage of the educational opportunities that are available to them. They have a better understanding of what education can do for them, greater respect for teachers and greater aspirations with regard to employment.
I think another issue is our class system, in which people who have regional dialects and non-standard grammar, are stereotyped, ignored and overlooked. Although this is changing it still does happen occasionally - the villain or ne'er do well in a drama often has a cockney accent for instance.