To an American, opposing Brexit would appear to be a no-brainer. The consequences of Brexit could include a renewed push for Scottish independence, new problems with the status of Northern Ireland, reduced foreign investment, and reduced growth, based on new difficulties in exporting to the EU. In exchange for that, the UK (or what would be left of it) would be able to stop the inflow of a relatively small number of immigrants, who are probably adding to growth, not decreasing it. That isn’t much of a bargain unless you are Donald Trump.
Boris Johnson’s London is arguably the most cosmopolitan city in the world. He always seemed to take pride in that. As a result, I can only attribute his support of Brexit to political ambition and opportunism; there is no other plausible explanation, and it does him no credit.